support preallocated VM images

Use fallocate where available to allocate file system blocks efficiently
when the VM is initially provisioned. This will give immediate feedback
if enough space isn't available. Also it may significantly improve
performance on writes to new blocks, and may even improve I/O
performance to prewritten blocks due to reduced fragmentation.

Also noted is the potential to subsume the less general
libvirt_sparse_logical_volumes option into the new more general
preallocate_images option.

Implements bp: preallocated-images
Change-Id: I4bbb8616416c9dca3a3a8894ebf764a0b6b8dbc9

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