https://bugzilla.redhat.com/show_bug.cgi?id=1161024

This way the device is in vmdef only if ret = 0 and the caller
(qemuDomainAttachDeviceFlags) does not free it.

Otherwise it might get double freed by qemuProcessStop
and qemuDomainAttachDeviceFlags if the domain crashed
in monitor after we've added it to vm->def.