When the guest writes a status of 0, the device should be reset. Add a
reset() callback for the transport layer to reset all queues and their
ioeventfd.

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>