daemon/parted: work around part table type misreporting by "parted"

"parted" incorrectly reports "loop" rather than "msdos" for the partition
table type, when the (fake) partition table comes from the "--mbr" option
of "mkfs.fat" (in dosfstools-4.2+), and the FAT variant in question is
FAT16 or FAT32. (See RHBZ#2026224.) Work this around by
- parsing the partition table ourselves, and
- overriding "loop" with "msdos" when appropriate.

Note that when the FAT variant is FAT12, "parted" fails to parse the fake
MBR partition table completely (see RHBZ#2026220), which we cannot work
around. However, FAT12 should be a rare corner case in libguestfs usage --
"mkfs.fat" auto-chooses FAT12 only below 9MB disk size, and even "-F 12"
can only be forced up to and including 255MB disk size.

Add the helper function "has_bogus_mbr" to the Utils module; we'll use it
elsewhere too.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1931821
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Richard W.M. Jones <rjones@redhat.com>
Message-Id: <20211125094954.9713-5-lersek@redhat.com>
[lersek@redhat.com: drop "fun" keyword, and use partial application, in
 the definition of "sec0at" [Rich]]

daemon/parted.ml

@@ -118,7 +118,12 @@ let part_get_parttype device =
   let fields = String.nsplit ":" device_line in
   match fields with
   | _::_::_::_::_::"loop"::_ -> (* If "loop" return an error (RHBZ#634246). *)
-     failwithf "%s: not a partitioned device" device
+     (* ... Unless parted failed to recognize the fake MBR that mkfs.fat from
+      * dosfstools-4.2+ created. In that case, return "msdos" for MBR
+      * (RHBZ#1931821).
+      *)
+     if Utils.has_bogus_mbr device then "msdos"
+     else failwithf "%s: not a partitioned device" device
   | _::_::_::_::_::ret::_ -> ret
   | _ ->
      failwithf "%s: cannot parse the output of parted" device