arm-autonomy/arm-autonomy-host-image-minimal: Use REQUIRED_DISTRO_FEATURES

Make use of REQUIRED_DISTRO_FEATURES rather than anonymous python for
determining that both arm-autonomy-host and xen are present in
DISTRO_FEATURES

Issue-Id: SCM-1862
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: Id21086ac4f4c45a89fa8fa8a2da28f04919f6b26
Signed-off-by: Jon Mason <jon.mason@arm.com>