qmp: Resume OOB-enabled monitor before processing the request

monitor_qmp_dispatcher_co() needs to resume the monitor if
handle_qmp_command() suspended it.  Two cases:

1. OOB enabled: suspended if mon->qmp_requests has no more space

2. OOB disabled: suspended always

We resume only after we processed the request.  Which can take a long
time.

Resume the monitor right when the queue has space to keep the monitor
available for out-of-band commands even in this corner case.

Leave the "OOB disabled" case alone.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210201161504.1976989-4-armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
[Trailing whitespace tidied up]