This is a preparation for qemu-ga-client which uses
QEMUMonitorProtocol class. The class tries to
negotiate capabilities on connect, however, qemu-ga
doesn't suppose it and fails.

This change makes the negotiation optional, though
it's still performed by default for compatibility.
Signed-off-by: NRyota Ozaki <ozaki.ryota@gmail.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>