qemu: added tests of the new default video type selection algorithm

The test case for x86_64 and neither cirrus nor vga capability is of the
xml2argv type because it actually fails to parse the XML at all [*] which
is something that xml2xml tests don't seem to handle. xml2argv test fails
to produce a qemu argv for this case which xml2argv tests can handle.

[*] This is a consequence of the decision not to have a fallback if the
obvious choices (cirrus and vga) aren't viable due to missing QEMU caps.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Pavel Mores <pmores@redhat.com>