qemu_hotplug: rename dev to match in qemuDomainDetachDeviceLive
    
    I'm about to add a second virDomainDeviceDef to this function that
    will point to the actual device in the domain object. while this is
    just a partially filled-in example of what to look for. Naming it
    match will make the code easier to follow.
    
    Signed-off-by: Laine Stump <laine@laine.org>
    ACKed-by: Peter Krempa <pkrempa@redhat.com>