libxl: fix cd insert/eject

libxl: fix cd insert/eject

When multiple disks are passed to libxl_wait_for_disk_ejects, watch the
correct disk paths.

Parse the backend type and backend domid from xenstore in
libxl_event_get_disk_eject_info.

libxl_event_get_disk_eject_info must return a valid string in
disk->vdev, while at the moment is free'ed before returning.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>