rbd: Allow RBD disks to be attached to a Instance
Better parse domain XMLs so network devices can be detached as well
CLOUDSTACK-2554: CloudStack fails to load XCP 1.6 hypervisors

Missing default constructor fails the agent manager reloading the XCP
resource on reboot of management server. This is fixed by using the
default constructor as do other Xen resources and include a new resource
ala XenServers for XCP1.6.

Signed-off-by: Prasanna Santhanam <tsp@apache.org>
(cherry picked from commit 806aeb990d28179ff532e97dbf64f87cd3d5ca34)

Conflicts:
	plugins/hypervisors/xen/src/com/cloud/hypervisor/xen/resource/XcpServerResource.java

Signed-off-by: Prasanna Santhanam <tsp@apache.org>