Fix XenServer's ability to boot xen type images
Fix XenServer's ability to boot xen type images

Fixes bug 1055431.

As the scheduler wants to boot a vm_mode=xen type of image, the host's
"supported_instances" capability is used for finding a good candidate.
In the Xapi case, this field was not populated.

This fix modifies the xapi xenhost plugin, so the Xen host capabilities
are returned back to the compute node, as "host_capabilities".
On the compute side, the mentioned information is used, to extract the
"supported_instances" information.

Change-Id: I2da11ab81f74b5b52e2c30832a694470978e21b0

n0ano/gantt,n0ano/gantt