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>