x86: Add a helper to allow tests to signal completion without a vmcall()

Putting a vmcall() at the end of a nested test isn't always convenient
to do, and isn't necessary.  Add a helper to allow the nested test
to make it possible to skip this requirement.

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Message-Id: <20220125203127.1161838-4-aaronlewis@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>