selftests: KVM: Add test for KVM_{GET,SET}_CLOCK

Add a selftest for the new KVM clock UAPI that was introduced. Ensure
that the KVM clock is consistent between userspace and the guest, and
that the difference in realtime will only ever cause the KVM clock to
advance forward.

Cc: Andrew Jones <drjones@redhat.com>
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20210916181555.973085-3-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>