monitor: Introduce monitor_fdset_*free

monitor: Introduce monitor_fdset_*free

Introduce new functions to remove and free no longer used fds and
fdsets.

We need those to decouple the remove/free routines from
monitor_fdset_cleanup() which will go away in the next patches.

The new functions:

- monitor_fdset_free/_if_empty() will be used when a monitor
  connection closes and when an fd is removed to cleanup any fdset
  that is now empty.

- monitor_fdset_fd_free() will be used to remove one or more fds that
  have been explicitly targeted by qmp_remove_fd().

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>