The regression is introduced by Commit da1eba6b, the new
codes with this commit doesn't reset "ret" to "-1" when
it fails on parsing the device XML (live device attachment)

This patch changes the codes to reset the "ret" and "-1",
and also changes the codes so that it don't modify "ret"
for condition checking.

How to reproduce:

% cat test.xml
<disk type='oops' device='disk'>
  <driver name='qemu' type='raw'/>
  <source file='/var/lib/libvirt/images/test.img'/>
  <target dev='vda' bus='virtio'/>
</disk>

% virsh attach-device $domain test.xml
Device attached successfully

The device attachment failed actually with error "unknown disk type 'oops'",
however, it reports success.