xen-xm: fix data loss in domain edit

xen-xm: fix data loss in domain edit

On CentOS5:
If "virsh edit $DOM" is used and an error happens (for example changing
any live cycle action to a non-existing value), libvirt forgets that
$DOM exists, since it is already removed from the internal hash tables,
which are used for domain lookup.
In once case (unreproducible) even the persistent configuration
/etc/xen/$DOM was deleted.

Instead of using the compound function xenXMConfigSaveFile() explicitly
use xenFomatXM() and virConfWriteFile() to distinguish between a failure
in converting the libvirt definition to the xen-xm format and a problem
when writing the file.

Signed-off-by: Philipp Hahn <hahn@univention.de>