Fix the warnings of 'produced an invalid screendump'

[qiankehan]

For the tp-libvirt cases running on libvirt-9.0.0 and qemu-kvm-7.2.0, it always print the warnings like this:

2023-02-17 16:59:57,907 env_process      L1976 WARNI| VM 'avocado-vt-vm1' produced an invalid screendump

The reason is that, since libvirt-v8.10.0-78-g538bee0e73(qemu: Prefer PNG for domain screenshot) and QEMU-v7.1.0-rc0~125^2~6, libvirt prefers using png as screenshot format, not ppm. However, the checking functions of avocado-vt expect ppm format.
So the testing code should expect the PNG format for libvirt >= 9.0.0 && qemu >= 7.2.0. And expect the PPM format otherwise.
@dzhengfy @chunfuwen