util: don't skip error prefixes when QMP is active

The vreport() function will print to HMP if available, otherwise
to stderr. In the event that vreport() is called during execution
of a QMP command, it will print to stderr, but mistakenly omit the
message prefixes (timestamp, guest name, program name).

This new usage of monitor_is_cur_qmp() from vreport() requires that
we add a stub to satisfy linking of non-system emulator binaries.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>