fish: Don't fail if some mountpoints in /etc/fstab are bogus (RHBZ#668574).

Fix guestfish (and other C tools) so that they ignore errors
when /etc/fstab contains bogus entries.

Update the documentation for inspect-get-mountpoints to emphasize
that callers must be aware of this when mounting the returned
values.

Add a regression test.

Update the example code ("inspect_vm") to reflect the way this
API ought to be called.

For more detail see:
https://bugzilla.redhat.com/show_bug.cgi?id=668574

python/examples/inspect_vm.py

@@ -44,7 +44,10 @@ for root in roots:
             return -1
     mps.sort (compare)
     for mp_dev in mps:
-        g.mount_ro (mp_dev[1], mp_dev[0])
+        try:
+            g.mount_ro (mp_dev[1], mp_dev[0])
+        except RuntimeError as msg:
+            print "%s (ignored)" % msg
 
     # If /etc/issue.net file exists, print up to 3 lines.
     filename = "/etc/issue.net"