libvirt: remove unused get_connection parameter from VIF driver

Previously the VIF driver would have to take different action
for OpenVSwitch depending on the libvirt version. Since we
bumped the min libvirt version this is no longer required, so
the get_connection parameter to the VIF driver is unused.

Blueprint: libvirt-driver-class-refactor
Change-Id: I1925a433a00f5ea3b65899ecb64d904045519340
Merge "libvirt: remove unused get_connection parameter from VIF driver"