Allow a VM to specify its migration type as a default choice. The valid
options are "default" (i.e. behave as now), "live" which forces a live
migration only, and "shutdown" which forces a shutdown migration only.
The new option is treated as a VM meta option and is set to default if
not found.
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