xen-blkback: report hotplug-status busy when detach is initiated but frontend device...

xen-blkback: report hotplug-status busy when detach is initiated but frontend device is busy.

In case of deferred detach xm/xend doesn't get notified about busy status
and has to wait timeout (default 100s) to report detach failure to user.
This behavior is sometime incorrectly interpreted as tool hang.

This patch updates the hotplug-status with busy so that xm gets notified
instead of timeout.

Orabug: 26072430
Signed-off-by: Niranjan Patil <niranjan.d.patil@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>