x86: fix padding in images

x86: fix padding in images

The previous rework of x86 image creation done in commit  cb007a7bf619
("x86: switch image generation to new code") broke the padding in
images.

 sda: p2 size 212992 extends beyond EOD, enabling native capacity
 sda: p2 size 212992 extends beyond EOD, truncated
 sd 0:0:0:0: [sda] Attached SCSI disk
 SQUASHFS error: squashfs_read_data failed to read block 0x2cc556
 unable to read id index table
 VFS: Cannot open root device "PARTUUID=ac5c9cd8-02" or unknown-block(8,2): error -5
 Please append a correct "root=" boot option; here are the available partitions:
 0800           19761 sda
  driver: sd
   0801           16384 sda1 ac5c9cd8-01

   0802            2865 sda2 ac5c9cd8-02

 Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,2)

Tested with x86/64 with Docker (squashfs), qemustart (ext4/squashfs) and
virtualbox (ext4/squashfs).

Ref: FS#2935
Fixes: cb007a7bf619 ("x86: switch image generation to new code")
Suggested-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Paul Spooren <mail@aparcar.org>
[commit subject and description tweaks]
Signed-off-by: Petr Štetiar <ynezz@true.cz>