Add new parameter to optionally enable Out-Of-Band for a QMP server.

An example command line:

  ./qemu-system-x86_64 -chardev stdio,id=char0 \
                       -mon chardev=char0,mode=control,x-oob=on

By default, Out-Of-Band is off.

It is not allowed if either MUX or non-QMP is detected, since
Out-Of-Band is currently only for QMP, and non-MUX chardev backends.

Note that the client STILL has to request 'oob' during qmp_capabilities;
in part because the x-oob command line option may disappear in the
future if we decide the capabilities negotiation is sufficient.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180326063901.27425-4-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[eblake: enhance commit message]
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>