tests: Set expected screenshot resolution
tests: Set expected screenshot resolution

According to video device type, set expected screenshot
resolution, 720x400 for vga device, 1280x800 for
virtio-gpu device.

Signed-off-by: howard zhang <90ec7750b2604c925990bbc10f98dfe9138a4c1a@arm.com>

kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt,kubevirt/kubevirt