virtio/net: Implement device and virtqueue reset

On exit_vq(), clean all resources allocated for the queue. When the device
is reset, clean the backend.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>