Stop including raw URL in cache directory path

There's too much room for path collisions, problematic characters, etc.
Instead, use the SHA-256 of the canonicalized URL as the cache directory,
and put the URL in a file inside it.
Increase minimum libvirt to 0.9.8

Any relevant Linux distribution release that has at least 0.9.4, has at
least 0.9.8.
vmnetx.execute: Silence spurious pylint warning
Display details on XML validation failure

If manifest or domain XML fails to validate, display details in the
Details turndown of FatalErrorWindow or on vmnetx-generate stdout.
Avoid relative import for consistency
Add domain XML schema from libvirt 0.9.8
vmnetx.generate: Validate domain XML against schema from old libvirt

Domain XML in generated images must be parsable by the oldest supported
libvirt.

Partial fix for #15.