qga: treat get-guest-fsinfo as "best effort"

In some container environments, there may be references to block devices
witnessable from a container through /proc/self/mountinfo that reference
devices we simply don't have access to in the container, and cannot
provide information about.

Instead of failing the entire fsinfo command, return stub information
for these failed lookups.

This allows test-qga to pass under docker tests, which are in turn used
by the CentOS VM tests.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220708153503.18864-2-jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>