Extend 'xm dumppolicy' to support Xen-API

Extend 'xm dumppolicy' to support Xen-API

I am extending 'xm dumppolicy' to be used via the Xen-API. For this
there are two new functions in the ACM policy class:
 - get the currently enforced policy including statistical data from
   the hypervisor
- get the ACM 'ssidref' of a Domain. Since this may be a ACM-specific
  variable or type (int) I put it into the ACM class.

I extended the Xen-API documentation with the two new functions.

Signed-off-by: Stefan Berger <Stefanb@us.ibm.com>