vhost-net: move stubs to a separate file

vhost-net: move stubs to a separate file

There is no reason for CONFIG_VHOST_NET to be specific to a single target;
it is a host feature that can be add to all targets, as long as they support
the virtio-net device.  Currently CONFIG_VHOST_NET depends on CONFIG_KVM,
but ioeventfd support is present in the core memory API and works with
other accelerators as well.

As a first step, move the vhost-net stubs to a separate file.  Later, they
will become conditional on CONFIG_VIRTIO_NET, which is not available in .c
files.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1543851204-41186-2-git-send-email-pbonzini@redhat.com>
Message-Id: <1550165756-21617-2-git-send-email-pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>