virtio_blk: add missing __dev{init,exit} markings

The remove member of the virtio_driver structure uses __devexit_p(), so
the remove function itself should be marked with __devexit.  And where
there be __devexit on the remove, so is there __devinit on the probe.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>