KVM: arm64: Validate register access for a Realm VM

The RMM only allows setting the GPRS (x0-x30) and PC for a realm
guest. Check this in kvm_arm_set_reg() so that the VMM can receive a
suitable error return if other registers are accessed.

Signed-off-by: Steven Price <steven.price@arm.com>
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Changes since v5:
 * Upper GPRS can be set as part of a HOST_CALL return, so fix up the
   test to allow them.