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).
