v2v: linux: pass the root device when using SUSE's mkinitrd

mkinitrd in SLE guests < 12 tries to get the root device by scanning the
fstab: this will fail, since v2v already remapped the devices from
hd*/sd* to vd* in the guest (including in its fstab).

Since we know what is the root device in the appliance, pass it to
mkinitrd directly, so it does not have to do guesswork.

Thanks to: Cédric Bosdonnat, for reporting the issue, and testing
the fix.

v2v/convert_linux.ml

@@ -517,7 +517,8 @@ let rec convert (g : G.guestfs) inspect source output rcaps =
           g#command [| "/sbin/mkinitrd";
                        "-m"; String.concat " " modules;
                        "-i"; initrd;
-                       "-k"; kernel.ki_vmlinuz |]
+                       "-k"; kernel.ki_vmlinuz;
+                       "-d"; inspect.i_root |]
         )
       )
       else if family = `Debian_family then (