qmp: fix __accept() in qmp.py

In QEMUMonitorProtocol, commit e9d17b6 removed the __sockfile creation
from __negotiate_capabilities(), which breaks _accept().  This causes
failures in qemu-io python based tests (i.e. tests 030 and 040).

This patch creates the sockfile in __accept() as well.

Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>