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

Parameter *mon is added, and local variable *mon added in previous patch
is removed. The caller readline_completion(), pass rs->mon as value, which
should be initialized in readline_init() called by monitor_init().

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>
