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

Now all completion functions do not use *cur_mon any more, instead
they use rs->mon. In short, structure ReadLineState decide where
the complete action would be taken now.

Tested with the case that qemu have two telnet monitors, auto
completion function works normal.

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>