Optimize VM image setup

These changes save over 5s on a VM setup phase of a typical
armv7l build. Instead of asking mkfs.ext4 to create a journal
(which is causing it to do flushed writes of lots of zero blocks)
and immediately deleting it afterwards, simply don't create it
initially. This alone saves 2s.

Prefering fallocate to make use of persistent preallocation
avoids fragmentation and small other fixes further speed up
the initial build setup phase by several seconds typically.
avoid a remount data=writeback on ext4

Won't work anyway, there is no journal. Silences a
warning.
Merge pull request #52 from dirkmueller/opt/vm_init_create

Optimize VM image setup