KVM: arm64: selftests: Test for setting ID register from usersapce

Add tests to verify setting ID registers from userspace is handled
correctly by KVM. Also add a test case to use ioctl
KVM_ARM_GET_REG_WRITABLE_MASKS to get writable masks.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231011195740.3349631-6-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>