monitor: refine monitor_find_completion()

In order to support sub command in auto completion, a reentrant function
is needed, so monitor_find_completion() is split into two parts. The
first part does parsing of user input which need to be done only once,
the second part does the auto completion job according to the parsing
result, which contains the necessary code to support sub command and
works as the reentrant function. The global "info_cmds" is still used
in second part, which will be replaced by sub command code later.

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>