arm-autonomy: xenguest-mgr: Add support for compressed partition images

xenguest-manager now can create guest partition
using compressed images like *.img.gz or *.img.bz2.

Change-Id: Ifaec0b7c301b2b316ebccdd0ca85083262f2714c
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>