Not all QEMU builds default to SDL graphics for their display.
Newer QEMU now has an explicit -sdl flag, which we can use to
explicitly request SDL intead of relying on the default. This
protects libvirt against unexpected changes in graphics default

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Probe for -sdl
  flag and use it if it is found
* tests/qemuhelptest.c: Add SDL flag to tests