QEMU always configures a VGA card. If no video card is included in
the libvirt XML, it is neccessary to explicitly turn off the default
using -vga none

* src/qemu/qemu_conf.c: Pass -vga none if no video card is configured
* tests/qemuargv2xmltest.c, tests/qemuxml2argvtest.c: Test for
  handling -vga none.
* tests/qemuxml2argvdata/qemuxml2argv-nographics-vga.args,
  tests/qemuxml2argvdata/qemuxml2argv-nographics-vga.xml: Test
  data files