monitor: Introduce MONITOR_USE_CONTROL flag

This flag will be set when Monitor enters "control mode", in
which the output will be defined by the QEMU Monitor Protocol.

This also introduces a macro to check if the flag is set.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
