monitor: Handle new and old style handlers

monitor: Handle new and old style handlers

This commit changes monitor_handle_command() to support old style
_and_ new style handlers.

New style handlers are protocol independent, they return their
data to the Monitor, which in turn decides how to print them
(ie. user protocol vs. machine protocol).

Converted handlers will use the 'user_print' member of 'mon_cmd_t'
to define its user protocol function, which will be called to print
data in the user protocol format.

Handlers which don't have 'user_print' defined are not converted
and are handled as usual.

Patchworks-ID: 35340
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>