improve libguestfs exception handling

* nova/virt/disk/vfs/guestfs.py (teardown): Handle specific
exceptions when tearing down so that (future) nova bugs
aren't masked.  Also handle AttributeError so that the
relatively new handle() and close() don't cause erroneous
debug logs on older versions of libguestfs.
(has_file): remove the last use of generic exception
catching and use the more specific RuntimeError.

Change-Id: Ia7345675d75c1c6727e9254791d44475c0ed9295
Merge "improve libguestfs exception handling"