KVM: arm/arm64: Add hook to finalize the vcpu configuration

Some aspects of vcpu configuration can't be completed inside
KVM_VCPU_INIT, but still change API behaviour visible to userspace.

Where such configuration choices affect the register list visible
to userspace, we will need to track whether we have made a
commitment to userspace regarding the list of vcpu registers.

This patch adds a new hook kvm_arm_vcpu_finalize() to capture this
commitment, along with a corresponding check
kvm_arm_vcpu_finalized().  We commit to the register list when
userspace reads it via KVM_GET_REG_LIST, or when the vcpu is fired
up via KVM_RUN.

kvm_arm_vcpu_finalize() is currently a no-op, but future patches
will amend this to handle SVE on arm64.

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