KVM: selftests: Introduce after_vcpu_run hook for dirty log test

Provide a hook for the checks after vcpu_run() completes.  Preparation
for the dirty ring test because we'll need to take care of another
exit reason.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012235.6063-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>