tools/libxc: Batch memory allocations for PV guests

tools/libxc: Batch memory allocations for PV guests

The current code for allocating memory for PV guests batches the
hypercalls to allocate memory by allocating 1024*1024 extents of order 0
at a time. To make this faster, first try allocating extents of order 9
(2 MiB) before falling back to the order 0 allocating if the order 9
allocation fails.

On my test machine this reduced the time to start a 128 GiB PV guest by
about 60 seconds.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>