Fixes a deadlock where the backend calls QEMU, while QEMU also calls the
backend simultaneously, both ends waiting for each other.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210204105232.834642-3-marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>