libxl: Don't overwrite errors from xenconfig
    
    When converting domXML from native, the libxl driver was overwriting
    useful errors from the xenconfig parsing code with a useless, generic
    error.  E.g. "internal error: parsing xm config failed" vs
    "internal error: config value usbdevice was malformed".  Remove the
    redundant (and useless) error reporting in the libxl driver.
    
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>