Makes page tables and virtual buffers able to be moved, but not copied,
making the interface more flexible.
Previously, with the destructor specified, but no move assignment or
constructor specified, they wouldn't be implicitly generated.
On DragonFly and NetBSD build fails with
src/common/virtual_buffer.cpp
src/common/virtual_buffer.cpp:16:10: fatal error: sys/sysinfo.h: No such file or directory
#include <sys/sysinfo.h>
^~~~~~~~~~~~~~~