Sometimes the only thing we need is the pointer to virDomainDiskDef and
having to call virDomainDiskIndexBy* APIs, storing the disk index, and
looking it up in the disks array is ugly. After this patch, we can just
call virDomainDiskBy* and get the pointer in one step.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>