xen-blkback: emit active physical device to xenstore

xen-blkback: emit active physical device to xenstore

The vbd physical-device is set by user-space writing to device's xenbus
path:
/local/domain/0/backend/vbd/1/51712/physical-device = "<major>:<minor>"

However, there is no way for the blkback driver to specify the current
state of the physical-device. Accordingly we add two new backend paths:

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

which specify the active physical device and any error incurred in
trying to instantiate a new physical device.

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>