hmp: Update current monitor only in handle_hmp_command()

hmp: Update current monitor only in handle_hmp_command()

The current monitor is updated relatively early in the command handling
code even though only the command handler actually needs it.

The current monitor will become coroutine-local later, so we can only
update it when we know in which coroutine the command will be exectued.
Move it to handle_hmp_command() where this information will be
available.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201005155855.256490-5-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>