xen-blkback: refactor backend_changed()

xen-blkback: refactor backend_changed()

Add xenbus_scan_be_params() to handle vbd param parsing. This gets
called from backend_changed().

Also place all the parameters (major, minor, cdrom, readonly) in struct
blkif_params. The readonly param is an integer replacement of mode --
this allows us to avoid tracking an additional pointer.

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>