virt-ppm_util: Just print a warning rather than fail if we have no PIL package.

As PIL is not available in several released OS, then import PIL will break
the all tests in autotest/virt-tests.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
virt_vm: Check the ppm_utils.Image before check_guest_bsod.

When we have no python-image package installed on host, importing
Image from PIL will fail and the ppm_utils.Image is None. In this
case, we should skip the ckeck_guest_bsod.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>