These are relatively complete, although only lightly tested.  Missing:

 - events
 - last_errno
 - user_cancel

This is a shorter and more convenient way to disable errors
temporarily across calls.

This commit models libvirt authentication events through the API,
adding one new event (GUESTFS_EVENT_LIBVIRT_AUTH) and several new
APIs:

  guestfs_set_libvirt_supported_credentials
  guestfs_get_libvirt_requested_credentials
  guestfs_get_libvirt_requested_credential_prompt
  guestfs_get_libvirt_requested_credential_challenge
  guestfs_get_libvirt_requested_credential_defresult
  guestfs_set_libvirt_requested_credential

See the documentation and example which shows how to use the new API.

This commit also changes existing calls to virConnectOpen* within the
library so that the new API is used.

Also included is an example (but not a test, because it's hard to see
how to automatically test the libvirt API).

Don't assume that if guestfs_case_sensitive_path returns NULL, that it
means the file does not exist.

The (previously undefined) behaviour of case_sensitive_path was that a
NULL return meant "either the file doesn't exist or some other error".
However in commit 973581780d8a006f336684fef6762801402d775d this was
changed so that if the last element of the path didn't exist, it was
assumed to be a new file and the (non-NULL) path of the new file is
returned.

This change breaks code (including in libguestfs) which tries to use
case_sensitive_path as a dual-purpose call to fix-up a path for
Windows and test if the file exists.  Such code should be rewritten so
that it explicitly tests for file existence after calling
case_sensitive_path.

I examined all the calls to case_sensitive_path in libguestfs and
modified them where necessary.