[libvirt] v3: Use posix_fallocate() on supported systems to allocate diskspace

[Amit Shah <amit shah redhat com>]

This patchset makes use of the posix_fallocate() call to allocate
chunks of files whenever needed if it's available.

We fallback to using safewrite() if it's not available.

mmap() could be used instead of safewrite too; I have a patch in case
someone is interested in seeing it.