conf,qemu: Replace iscsisrc fields with virStorageSourcePtr
    
    Rather than picking apart the two pieces we need/want (path, hosts,
    and auth)- let's allocate/use a virStorageSourcePtr for iSCSI storage.
    
    The end result is that qemuBuildSCSIiSCSIHostdevDrvStr doesn't need
    to "fake" one for the qemuBuildNetworkDriveStr call.