qapi: better document NVMe blockdev @device parameter

qapi: better document NVMe blockdev @device parameter

Mention that this is a PCI device address & give the format it is
expected in. Also mention that it must be first unbound from any
host kernel driver.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit ecaf647f300d69271c245a67adca9c3a89d9463a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>