qemu: fix RBD attach regression

qemu: fix RBD attach regression

I have been testing libvirt v1.0.0 for deployment within my
organization, and in the process discovered what appears to be a bug
that breaks virsh attach-device, when attaching an RBD volume to an
instance. First, here is the error presented, with v1.0.0 (this worked
in v0.10.2):

[root@host ~]# virsh attach-device W5APQ8  G84VV1.xml
error: Failed to attach device from G84VV1.xml
error: cannot open file 'dc3-1-test/G84VV1': No such file or directory

Using git bisect, I narrowed the problem down to this as the first
commit to break this setup:

4d34c92947e8cf9e9bedfa227ada1d2dba92d68a is the first bad commit