Revert "Switch qxl guest video to cirrus on Ubuntu 12.04"

Ubuntu 12.04 is no longer supported.

This reverts commit 4c096f1dcd453c42de4356fce4cfbf361ed85870.
Always use SPICE viewer

All supported distribution versions now support SPICE.
ui.view: Drop VNCWidget
ui.view: Drop _ViewerWidget superclass
util: Bump minimum libvirt version to 0.9.12

for Debian Wheezy.
Update bundled libvirt domain XML schema to 0.9.12
domain: Stop stripping metadata from domain XML

All supported libvirt versions should be able to handle it.
schema/domain: Require unicast MAC addresses

From libvirt 0.9.12 schema.
domain: Allow memory unit attribute

It's supported by libvirt >= 0.9.11.
domain: Allow vcpu placement="static"
domain: Allow cpu model fallback="allow"
domain: Allow cpu mode="custom"
domain: Allow address target attribute
domain: Document libvirt versions introducing new XML items

that we remove in make_backward_compatible().