[libvirt] Fix slow storage file allocation with O_DSYNC

Hi. By opening storage file with O_DSYNC before allocating disk blocks for it we made the whole operation terribly slow when the allocation is done using posix_fallocate() on a filesystem which does not support fallocate(), i.e. anything but ext4. And by terribly slow I mean 45 minutes (RHEL5) to 10+ hours (Fedora Rawhide) for a 12GB storage. To fix this issue, we have two options. Either avoid using fallocate() emulation inside posix_fallocate() or stop using O_DSYNC. I prepared a patch for each of these options and they will follow as replies to this introduction. Some numbers for the same 12GB file: - libvirt's internal fallocate emulation using - mmap ~3.5 minutes (will be used whenever mmap() is present) - write ~5 minutes - fsync() instead of O_DSYNC - ~3.5 minutes So basically the two options are equivalent wrt to time consumption. The second one seems to be less hacky at the expense of non-lienar behavior which makes progress reporting less useful. Jirka