NVMe: Handling devices incapable of I/O

This is a minor refactor for handling devices that are incapable of IO.
The driver previously used special error codes to know that IO queues
are unavailable, but we have an online queue count now.

This also fixes an issue where the driver successfully sets the queue
count, but either is unable to allocate an IO queue or the device can't
create one for some reason.

If the driver can successfully enable the device and get responses to
admin commands, the driver will bring up a character device for managment
but not create block devices.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>