utils_test API Split

Reimplementing pull request #769, in a more consistent way (each patch is a change complete in itself).

This splits the utils_test module effectively in 3 modules:

utils_test: Generic test utilites
utils_test.qemu: QEMU test specific utilities
utils_test.libvirt: libvirt test specific utilities

The modules were cleaned up, old API usage was updated, so we have shorter modules, and will help other subtests to implement specific utilities.