inspect: check presence of BIOS boot partition to handle BIOS+GPT setup

If the guest uses BIOS firmware but GPT partitioned disk, v2v inspection
might fail to correctly detect the firmware being used by the source machine.
Namely, consider the following GPT partition table:

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048            4095   1024.0 KiB  EF02
   2            4096          528383   256.0 MiB   EF00
   3          528384       125827071   59.7 GiB    8300
   4       125827072       134215679   4.0 GiB     8200

where partition 1 is a BIOS boot partition (code 0xEF02, GPT partition
entry with GUID 21686148-6449-6E6F-744E-656564454649), and partition 2
is an EFI System partition (code 0xEF00, GPT partition entry with GUID
C12A7328-F81F-11D2-BA4B-00A0C93EC93B).  The former is commonly used to
store the bootloader in BIOS+GPT machines.  The latter is used by UEFI
powered machines.

Normally an OS installer wouldn't put those two together: if the disk is
being partitioned in GPT, there's either BIOS boot partition (if BIOS
firmware is used) or EFI System partition (if UEFI firmware is used).
However, GRUB2 will deal with such "busted" layout just fine, so this
configuration may exist. If it is the case, v2v inspection will detect the
presence of an EFI system partition and mistakenly mark the system as UEFI.

So let's prioritize Bios boot partition over ESP.  As discussed in [1],
this solution is not entirely bulletproof.  It will work in case a GPT
disk was first used in a UEFI machine, and then put into a BIOS machine
with an ESP partition remaining on it.  It won't work in the opposite case,
i.e. when a GPT disk is moved from BIOS machine to UEFI machine with
BIOS boot partition remaining on it. However, it's better to prioritize
something, and the latter case is less probable than the former.

[1] https://listman.redhat.com/archives/libguestfs/2022-December/030401.html

Co-authored-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Message-Id: <20221221163831.423551-3-andrey.drobyshev@virtuozzo.com>
[lersek@redhat.com: fix typos in the commit message]

convert/inspect_source.ml

@@ -219,6 +219,9 @@ and list_applications g root = function
 (* See if this guest could use UEFI to boot.  It should use GPT and
  * it should have an EFI System Partition (ESP).
  *
+ * If the guest has BIOS boot partition present, this is likely a BIOS+GPT
+ * setup, so [BIOS] is returned.
+ *
  * If it has ESP(s), then [UEFI devs] is returned where [devs] is the
  * list of at least one ESP.
  *
@@ -233,24 +236,31 @@ and get_firmware_bootable_device g =
          debug "%s (ignored)" msg;
          false
   in
-  let accumulate_partition esp_parts part =
+  let accumulate_partition (esp_parts, bboot) part =
     let dev = g#part_to_dev part in
     if parttype_is_gpt dev then
       let partnum = g#part_to_partnum part in
       let part_type_guid = g#part_get_gpt_type dev partnum in
       match part_type_guid with
       (* EFI system partition *)
-      | "C12A7328-F81F-11D2-BA4B-00A0C93EC93B" -> part :: esp_parts
-      | _ -> esp_parts
-    else esp_parts
+      | "C12A7328-F81F-11D2-BA4B-00A0C93EC93B" -> part :: esp_parts, bboot
+      (* BIOS boot partition *)
+      | "21686148-6449-6E6F-744E-656564454649" -> esp_parts, true
+      | _ -> esp_parts, bboot
+    else esp_parts, bboot
   in
 
-  let esp_partitions =
-    Array.fold_left accumulate_partition [] (g#list_partitions ()) in
+  let esp_partitions, bios_boot =
+    Array.fold_left accumulate_partition ([], false) (g#list_partitions ()) in
 
-  match esp_partitions with
-  | [] -> I_BIOS
-  | partitions -> I_UEFI partitions
+  (* If there's a BIOS boot partition present (0xef02 type for gdisk,
+   * "bios_grub" flag for parted), then this is likely a BIOS+GPT setup.
+   * In this case we prioritize BIOS boot partition and detect BIOS firmware,
+   * no matter how many ESPs we've found.
+   *)
+  match esp_partitions, bios_boot with
+  | _ :: _, false -> I_UEFI esp_partitions
+  | _ -> I_BIOS
 
 (* If some inspection fields are "unknown", then that indicates a
  * failure in inspection, and we shouldn't continue.  For an example