The attached patch adds a new dom0_op, DOM0_GETDOMAININFOLIST. This

The attached patch adds a new dom0_op, DOM0_GETDOMAININFOLIST.  This
operation allows retrieval of the domain info structures for all domains
in one hypercall.

Using a small test program, on a system with 97 domains with
non-contiguous domain IDs, I found that with this hypercall I could
retrieve the full array of info structures 12840 times per second, an
improvement from 2380 times per second with the DOM0_GETDOMAININFO op.

The patch also adds a libxc wrapper for this new hypercall.

Signed-off-by: Josh Triplett <josht@us.ibm.com>