parse_libvirt_xml: look for manual firmware in "/domain/os/loader/@type"

According to [1], there're different ways to specify which firmware is
to be used by a libvirt-driven VM.  Namely, there's an automatic
firmware selection, e.g.:

    ...
    <os firmware='(bios|efi)'>
    ...

and a manual one, e.g.:

    ...
    <os>
        <loader readonly='yes' type='pflash'>/usr/share/OVMF/OVMF_CODE.fd</loader>
        ...
    </os>
    ...

with the latter being a way to specify UEFI firmware.  So let's add this
search path as well when parsing source VM's libvirt xml.

[1] https://libvirt.org/formatdomain.html#bios-bootloader

Co-authored-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Originally-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Message-Id: <20221220150133.403120-1-andrey.drobyshev@virtuozzo.com>

input/parse_libvirt_xml.ml

@@ -446,12 +446,23 @@ let parse_libvirt_xml ?conn xml =
     done;
     List.rev !nics in
 
-  (* Firmware. *)
+  (* Firmware.
+   * If "/domain/os" node doesn't contain "firmware" attribute (automatic
+   * firmware), we look for the presence of "pflash" in
+   * "/domain/os/loader/@type" attribute (manual firmware), with the latter
+   * indicating the UEFI firmware.
+   * See https://libvirt.org/formatdomain.html#bios-bootloader
+   *)
   let firmware =
     match xpath_string "/domain/os/@firmware" with
     | Some "bios" -> BIOS
     | Some "efi" -> UEFI
-    | None | Some _ -> UnknownFirmware in
+    | Some _ -> UnknownFirmware
+    | None -> (
+        match xpath_string "/domain/os/loader/@type" with
+        | Some "pflash" -> UEFI
+        | _ -> UnknownFirmware
+      ) in
 
   (* Check for hostdev devices. (RHBZ#1472719) *)
   let () =