monitor: avoid use of global *cur_mon in cmd_completion()

A new local variable *mon is added in monitor_find_completion()
to make compile pass, which will be removed later in
conversion patch for monitor_find_completion().

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