virtio: add debugfs infrastructure to allow to debug virtio features

Currently there is no way for user to set what features the driver
should obey or not, it is hard wired in the code.

In order to be able to debug the device behavior in case some feature is
disabled, introduce a debugfs infrastructure with couple of files
allowing user to see what features the device advertises and
to set filter for features used by driver.

Example:
$cat /sys/bus/virtio/devices/virtio0/features
1110010111111111111101010000110010000000100000000000000000000000
$ echo "5" >/sys/kernel/debug/virtio/virtio0/filter_feature_add
$ cat /sys/kernel/debug/virtio/virtio0/filter_features
5
$ echo "virtio0" > /sys/bus/virtio/drivers/virtio_net/unbind
$ echo "virtio0" > /sys/bus/virtio/drivers/virtio_net/bind
$ cat /sys/bus/virtio/devices/virtio0/features
1110000111111111111101010000110010000000100000000000000000000000

Note that sysfs "features" now already exists, this patch does not
touch it.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>