The dom destroy path is doing a dom_get on a non-existent domain to

The dom destroy path is doing a dom_get on a non-existent domain to
ensure it is non-existent.  This changes throws an explicit exception
when xc_domain_getinfo returns an error, instead of triggering an
internal python error.  It then handles the exception in dom_get by
returning None, which callers already expect to mean failure.

Signed-off-by: Robert Read <robert@xensource.com>