Use the new "provisioned" memory field, instead of the "allocated"
memory field, to determine the optimal node when using the "mem"
migration selector. This will take into account non-running VMs in the
calculation as well as running VMs.
Allow the user to specify other, non-raw files and upload them,
performing a conversion with qemu-img convert and a temporary block
device as a shim (since qemu-img can't use FIFOs).
Also ensures that the target volume exists before proceeding.
Addresses #68
Joshua Boniface