Right now *-headless and *-graphics tests are using different
quoting styles, which results in the diff between them being
basically useless, whereas we would like it to be possible to
compare these files directly and easily spot the differences.

Convert all *-graphics tests to single quotes, which is the
style libvirt itself uses when formatting XML: this is a fact
that will come in handy later.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>