qemu: eliminate nested switch, simplify code
    
    qemuBuildHostNetStr had a switch-within-a-switch where both were
    looking at the same variable. This was apparently to take advantage of
    code common to three different cases (while also taking care of some
    code that was different). However, there were only 2 lines common to
    all, one of those can be eliminated by merging it into the
    virAsprintfs that are in each case. On top of that, all the extra
    empty cases cause Coverity complaints (because they are unreachable),
    but absence of the empty cases causes a compile error due to
    "enumeration value not handled in switch".
    
    The solution is to just make each toplevel case independent, folding
    in the common code to each.