virtio/blk: Reset virtqueue

Move pthread creation to init_vq, and kill the thread in exit_vq.
Initialize the virtqueue states at runtime.

All in-flight I/O is canceled with the virtqueue pthreads, except for AIO
threads, but after reading the code I'm not sure if AIO has ever worked
anyway.

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>