xen_disk: cope with missing xenstore "params" node

xen_disk: cope with missing xenstore "params" node

When disk is a cdrom and the drive is empty the "params" node in
xenstore might be missing completely: cope with it instead of
segfaulting.

Updated in v2:

- actually removed the strchr(blkdev->params, ':') that caused the
segfault;

- free all the allocated strings from xenstore before returning;

Updated in v3:

- set blkdev fields to NULL after free'ing them.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>