KVM: arm64: selftests: Introduce vcpu_width_config

Introduce a test for aarch64 that ensures non-mixed-width vCPUs
(all 64bit vCPUs or all 32bit vcPUs) can be configured, and
mixed-width vCPUs cannot be configured.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220329031924.619453-3-reijiw@google.com