ui: Don't rely on OVIRT_BOOTIF

Previously the UI relied on OVIRT_BOOTIF to determin the primary
interface. This lead to problems because OVIRT_BOOTIF is erased at each
boot.
Now the primary interface is determined by looking at the system wide
ifcfg files.

Change-Id: I7ace62b39d37d2b81054ac8b676eb3448acb1c09
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>