KVM: arm: Move detection of invalid VM type bits to generic code

Currently, the detection of invalid bits in the KVM_CREATE_VM type
argument is done in the kvm_arm_setup_stage2() backend.

In order to make it easier to add type flags with independent
meanings, this patch moves the logic for rejecting invalid bits to
kvm_arch_init_vm().  Backend functions are now responsible for
clearing bits they know about from type, so that if any remain
after all such functions are called, we know they are invalid.

No functional change.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>