scripts/vmstate-static-checker: Fix deprecation warnings with latest argparse

scripts/vmstate-static-checker: Fix deprecation warnings with latest argparse

The argparse.FileType() type has been deprecated in the latest argparse
version (e.g. the one from Fedora 43), now causing the test_bad_vmstate
functional test to fail since there are unexpected strings in the output.
Change the script to use pathlib.Path instead to fix the test_bad_vmstate
test and to be prepared for the future when the deprecated FileType gets
removed completely.

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Link: https://lore.kernel.org/r/20251030092638.39505-1-thuth@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>