virttest.ppm_utils: Skip invalid ppm files

When testing the code under RHEL5, we've noticed that Windows
install tests would fail with:

Unhandled IOError: cannot identify image file [context: waiting for installation to finish]

Let's handle image comparison more carefully, returning
false if the screenshot file had any problems.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>