vhost-net: compile it on all targets that have virtio-net.

vhost-net: compile it on all targets that have virtio-net.

This shows a preexisting bug: if a KVM target did not have virtio-net enabled,
it would fail with undefined symbols when vhost was enabled.  This must now
be fixed, lest targets that have no virtio-net fail to compile.

Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-Id: <1543851204[email protected]>
Message-Id: <1550165756[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>