util: clang is failing to compile due to unused variables.
    
    After some recent patches, clang is throwing some errors related to
    unused variables. This is not happening when we use GCC with -Werror
    enabled. Only clang reports this warning.
    
    make[3]: Entering directory '/home/julio/Desktop/virt/libvirt/src'
      CC       util/libvirt_util_la-virscsivhost.lo
      CC       util/libvirt_util_la-virusb.lo
      CC       util/libvirt_util_la-virmdev.lo
    util/virmdev.c:373:36: error: unused variable 'ret' [-Werror,-Wunused-variable]
        VIR_AUTOPTR(virMediatedDevice) ret = virMediatedDeviceListSteal(list, dev);
                                       ^
    1 error generated.
    Makefile:11579: recipe for target 'util/libvirt_util_la-virmdev.lo' failed
    make[3]: *** [util/libvirt_util_la-virmdev.lo] Error 1
    make[3]: *** Waiting for unfinished jobs....
    util/virscsivhost.c:112:37: error: unused variable 'tmp' [-Werror,-Wunused-variable]
        VIR_AUTOPTR(virSCSIVHostDevice) tmp = virSCSIVHostDeviceListSteal(list, dev);
                                        ^
    1 error generated.
    Makefile:11411: recipe for target 'util/libvirt_util_la-virscsivhost.lo' failed
    make[3]: *** [util/libvirt_util_la-virscsivhost.lo] Error 1
    util/virusb.c:511:31: error: unused variable 'ret' [-Werror,-Wunused-variable]
        VIR_AUTOPTR(virUSBDevice) ret = virUSBDeviceListSteal(list, dev);
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Reviewed-by: John Ferlan <jferlan@redhat.com>