Some tools, such as virt-manager, prefers having the default USB
controller explicit in the XML document. This patch makes sure there
is one. With this patch, it is now possible to switch from USB1 to
USB2 from the release 0.9.1 of virt-manager.
Fix tests to pass with this change.
QEMU 0.13 introduced cache=unsafe for -drive, this patch exposes
it in the libvirt layer.
* Introduced a new QEMU capability flag ($prefix_CACHE_UNSAFE),
as even if $prefix_CACHE_V2 is set, we can't know if unsafe
is supported.
* Improved the reliability of qemu cache type detection.
