monitor: split off monitor_data_init()

In qmp_human_monitor_command(), the monitor need to initialized for
basic functionalities, and later more init code will be added, so
split off this function. Note that it is different with QMP mode
monitor which accept json string from monitor's input,
qmp_human_monitor_command() retrieve the human style command from
QMP input, then send the command to a normal mode monitor.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
