vhost: drop virtqueues only with built-in virtio driver

vhost: drop virtqueues only with built-in virtio driver

Commit e29109323595beb3884da58126ebb3b878cb66f5 ("vhost: destroy unused
virtqueues when multiqueue not negotiated") broke vhost-scsi by removing
virtqueues when the virtio-net-specific VIRTIO_NET_F_MQ feature bit is
missing.

The vhost_user.c code shouldn't assume all devices are vhost net device
backends.  Use the new VIRTIO_DEV_BUILTIN_VIRTIO_NET flag to check
whether virtio_net.c is being used.

This fixes examples/vhost_scsi.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>