Add support for nullio ramdisks

Added in 3.10-rc2.

Signed-off-by: Andy Grover <agrover@redhat.com>
Don't allow reading wwn from unconfigured SOs

I don't think anybody does this but if they try, they should have to
catch the exception.

Signed-off-by: Andy Grover <agrover@redhat.com>
Add _configure() to StorageObject

Call it from derived objs' _configure method. This allows us to do common
post-enablement tasks in one place. One example is setting WWN, although
a no-op override method is needed for pscsi.

If we want to handle PR APTPL in StorageObject we also need this, since it
needs WWN set.

Signed-off-by: Andy Grover <agrover@redhat.com>
Add support for restoring PR APTPL info

While we pursue a better solution upstream, this will properly restore
the aptpl info on SO creation.

Signed-off-by: Andy Grover <agrover@redhat.com>