tests/qtest/display-vga-test: Add proper checks if a device is available

tests/qtest/display-vga-test: Add proper checks if a device is available

display-vga-test currently tries to guess the usable VGA devices
according to the target architecture that is used for the test.
This of course does not work if QEMU has been built with the
"--without-default-devices" configure switch. To fix this, use the
qtest_has_device() function for the decision instead. This way
we can also consolidate most of the test functions into one single
function (that takes a parameter with the device name now), except
for the multihead test that tries to instantiate two devices and
thus is a little bit different.

Message-Id: <20230130104446.1286773-4-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>