libxl: fix return value error Attach|DetachDeviceFlags

libxl: fix return value error Attach|DetachDeviceFlags

Code logic in libxlDomainAttachDeviceFlags and libxlDomainDetachDeviceFlags
is wrong with return value in error cases.

'ret' was being set to 0 if 'flags & VIR_DOMAIN_DEVICE_MODIFY_CONFIG' was
false. Then if something like virDomainDeviceDefParse() failed in the
VIR_DOMAIN_DEVICE_MODIFY_LIVE logic, the error would be reported but the
function would return success.

Signed-off-by: Chunyan Liu <cyliu@suse.com>