ArmVirtPkg: Xen: Add a NULL implementation of ArmCcaConfigureMmio

To support Arm CCA, a hook function ArmCcaConfigureMmio () has
been added to the ArmVirtMemInfoLib library.

Since, Arm CCA has not been enabled for the Xen guest firmware,
update the XenVirtMemInfoLib library to add a NULL implementation
for ArmCcaConfigureMmio () that returns RETURN_UNSUPPORTED.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>