libvirt: remove use of CONF.libvirt.virt_type in vif.py

The vif.py class imports the CONF.libvirt.virt_type parameter
from the main driver.py file. This sets up a circular dependency
which prevents driver.py importing vif.py in the top of the file,
since the CONF.libvirt.virt_type parameter won't have been
registered yet.

It is generally bad design practice to rely on global variables,
which is effectively what the CONF.* parameters are, so change
the VIF driver get_config() API to accept 'virt_type' as a
parameter instead.

Related-bug: #1302796
Change-Id: I023851df96640eeeb54124add8cc5c725b8d13be
Merge "libvirt: remove use of CONF.libvirt.virt_type in vif.py"