Add management of the pvcprov database with SQLAlchemy, to allow
seamless management of the database. Add automatic tasks to the postinst
of the API to execute these migrations.
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