Fix build in qemu_command

Currently, build with clang fails with:

  CC       qemu/libvirt_driver_qemu_impl_la-qemu_command.lo
qemu/qemu_command.c:6580:58: error: implicit conversion from enumeration type
'virMemAccess' to different enumeration type 'virTristateSwitch'
[-Werror,-Wenum-conversion]
        virTristateSwitch memAccess = def->cpu->cells[i].memAccess;
                          ~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~^~~~~~~~~
1 error generated.

Fix that by using virMemAccess instead of virTristateSwitch.

src/qemu/qemu_command.c

@@ -6577,7 +6577,7 @@ qemuBuildNumaArgStr(virQEMUDriverConfigPtr cfg,
     for (i = 0; i < def->cpu->ncells; i++) {
         int cellmem = VIR_DIV_UP(def->cpu->cells[i].mem, 1024);
         def->cpu->cells[i].mem = cellmem * 1024;
-        virTristateSwitch memAccess = def->cpu->cells[i].memAccess;
+        virMemAccess memAccess = def->cpu->cells[i].memAccess;
 
         VIR_FREE(cpumask);
         VIR_FREE(nodemask);