Commit 55320c23 introduced a new test for VNC to test if
vnc_auto_unix_socket is set in qemu.conf, but forget to enable it in
qemuxml2argvtest.c.

This patch also moves the code in qemuxml2xmltest.c next to other VNC
tests and refactor the test so we also check the case for parsing active
XML.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>