xen-blkback: implement swapping of active vbd

xen-blkback: implement swapping of active vbd

Currently we disallow any change of major:minor of the vbd once created.
The danger is in the user switching backends where the contents of the
backend device are dissimilar.
However, changing the vbd can be quite useful -- for instance by switching
from a backend which is not multi-pathed (or raid'd) to one that is.

As for blkback itself, it is used purely as a passthrough device so there
is no state outside struct vbd which would be impacted with this change.

This patch allows a vbd swap, communicating the state of the swap via the
xenbus paths:

.../vbd/<domain>/51712/oracle/active-physical-device = "<major>:<minor>"
.../vbd/<domain>/51712/oracle/physical-device-change-status = "<error>"

Orabug: 28651655

Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Bhavesh Davda <bhavesh.davda@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>