Fix domain exit actions that contain hyphen

Fix domain exit actions that contain hyphen

Domain exit actions that contain a hyphen (e.g. rename-restart) were
not being detected properly when xm is configured to use xenapi.
Domain config containing on_crash=3D"rename-restart" results in

xen53:~ # xm new /tmp/domU.config
Using config file "/tmp/domU.config".
Unexpected error: <type 'exceptions.TypeError'>

This patch fixes the raised exception and at the same time
handles the replacement of hyphen with underscore properly.

Signed-off-by: Jim Fehlig <jfehlig@novell.com>