[libvirt] Use posix_fallocate() to allocate disk space

These patches introduce safezero() to allocate chunks for a file and write zeroes to them. Not yet ready for merge; I don't understand the configure system yet and don't know how to test for posix_fallocate() availability. The second patch introduces a new variable, track_allocation_progress, which perhaps belongs somewhere else. Pointers for both these problems appreciated. Thanks, Amit