arm-autonomy/xenguest-mkimage: fix DISK_DEVICE setting

Previously '--disk-device' was set according to DISK_SIZE value,
set with '--disk-size' option, which was not quite accurate,
because disk device is more likely to be '/dev/sda2' rather than '1'.
This patch also fixes typo in help message.

Change-Id: I19aa72f3adb32cd786bb570023b6b40cfc5633bf
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-autonomy/recipes-extended/xenguest/files/xenguest-mkimage

@@ -127,7 +127,7 @@ Disk configuration
 --disk-rm-part=ID       remove partition ID from the guest disk
 --disk-add-file=SRC:DST add file SRC as disk file DST in the xenguest image.
                          DST can then be used as a partition CONTENT.
---disk--rm-file=DST     remove disk file DST from the xenguest image.
+--disk-rm-file=DST     remove disk file DST from the xenguest image.
 EOF
 }
 
@@ -746,7 +746,7 @@ for arg in "${@}"; do
                 ${IMAGE_TMPDIR}/disk.cfg
             ;;
         --disk-device=*)
-            sed -i "s/DISK_DEVICE=.*/DISK_SIZE=\"${optarg}\"/" \
+            sed -i "s@DISK_DEVICE=.*@DISK_DEVICE=\"${optarg}\"@" \
                 ${IMAGE_TMPDIR}/disk.cfg
             ;;
         --disk-add-part=*)