[libvirt] Use posix_fallocate() to allocate disk space

[Amit Shah <amit shah redhat com>]

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