qemu: Don't leak reference to virQEMUDriverConfigPtr

qemu: Don't leak reference to virQEMUDriverConfigPtr

https://bugzilla.redhat.com/show_bug.cgi?id=1011330 (case D)

qemuProcessStart created two references to virQEMUDriverConfigPtr before
calling fork():

    cfg = virQEMUDriverGetConfig(driver);
    ...
    hookData.cfg = virObjectRef(cfg);

However, the child only unreferenced hookData.cfg and the parent only
removed the cfg reference. That said, we don't need to increment the
reference counter when assigning cfg to hookData. Both the child and the
parent will correctly remove the reference on cfg (the child will do
that through hookData).

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>