virtio: Add get_vq() callback

To ease future changes to the core, replace get_pfn_vq() with get_vq().
This way adding new generic operation on virtqueues won't require
modifying every virtio device.

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>