QMP: Don't use do_info()

QMP: Don't use do_info()

Since its inception, QMP has been using HMP's do_info() function
to run query commands.

This was a bad choice, as it made do_info() more complex and
contributed to couple QMP and HMP.

This commit fixes that by doing the following changes:

  1. Introduce qmp_find_query_cmd() and use it to directly lookup
     the info_cmds table

  2. Introduce qmp_call_query_cmd() so that QMP code is able
     to call query handlers without using do_info()

  3. Drop do_info() usage (via monitor_find_command("info"))

We need all the three changes in one shot so that we don't break
the calling of query commands in QMP.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>