arm64: KVM: guest one-reg interface

Let userspace play with the guest registers.

Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Git-commit: 2f4a07c5f9fe4a5cdb9867e1e2fcab3165846ea7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>