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.