arm-autonomy/xenguest: Add guestcount support

Add guestcount parameter to ARM_AUTONOMY_HOST_IMAGE_EXTERN_GUESTS,
to create multiple guests with the same config. Symlinks are created
in the host rootfs pointing to the original xenguest file

Parameters for ARM_AUTONOMY_HOST_IMAGE_EXTERN_GUESTS are now
retrieved using python regex rather than split, so that they can
appear in any order

Issue-Id: SCM-1863
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: Ifa5f5ab3b284815427c94979b3f1a5eed592ec3c
Signed-off-by: Jon Mason <jon.mason@arm.com>