use posix_fallocate() for preallocated hard disks when possible

[bjacke]

when creating a new hard disk it would be good if preallocated (fixes size) hard disks would be created via posix_fallocate when that call is availbale. On recent Linux systems this call will allocate the whole space very fast and efficiently when XFS or ext4 is being used as filesystem.

[bjacke]

ping. creating preallocated images ist still a pain. Why don't you make use of the filesystem doing that work efficiently in less than a second? Would also be a trivial change to do.

[Donuts]

Yep, this feature should be trivial to implement and would be a huge time-saver when creating non-expanding VDIs. Even creating a small 10GB VDI on a USB 2.0 hard drive takes several minutes. Using fallocate() it would be almost instantaneous.