libxl: correct allocation size in libxl_list_vm

libxl: correct allocation size in libxl_list_vm

*ptr has type libxl_vminfo not libxl_domid, so correct calloc call.

This the second instance of this bug I've noticed recently, I did a
quick audit of other similar uses of sizeof(...) and all I spotted
were a couple of harmlessly reversed calloc arguments. It's a pretty
strong argument for "foo = ..alloc(sizeof(*foo))" rather than
"alloc(sizeof(foos_type))" though...

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>