lib: Autodetect backing format for qemu-img create -b

qemu 6.1 has decided to change qemu-img create so that a backing
format (-F) is required if a backing file (-b) is specified.  Since we
don't want to change the libguestfs API to force callers to specify
this because that would be an API break, autodetect it.

This is similar to commit c8c181e ("launch: libvirt: Autodetect
backing format for readonly drive overlays").

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1998820

lib/create.c

@@ -255,6 +255,7 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
                    const struct guestfs_disk_create_argv *optargs)
 {
   const char *backingformat = NULL;
+  CLEANUP_FREE char *backingformat_free = NULL;
   const char *preallocation = NULL;
   const char *compat = NULL;
   int clustersize = -1;
@@ -270,6 +271,14 @@ disk_create_qcow2 (guestfs_h *g, const char *filename, int64_t size,
       return -1;
     }
   }
+  else if (backingfile) {
+    /* Since qemu 6.1, qemu-img create has requires a backing format (-F)
+     * parameter if backing file (-b) is used (RHBZ#1998820).
+     */
+    backingformat = backingformat_free = guestfs_disk_format (g, backingfile);
+    if (!backingformat)
+      return -1;
+  }
   if (optargs->bitmask & GUESTFS_DISK_CREATE_PREALLOCATION_BITMASK) {
     if (STREQ (optargs->preallocation, "off") ||
         STREQ (optargs->preallocation, "sparse"))