The memory API supports 64-bit buses (e.g. PCI).  A size on such a bus cannot
be represented with a 64-bit data type, if both 0 and the entire address
space size are to be represented.  Futhermore, any address arithemetic may
overflow and return unexpected results.

Introduce a 128-bit signed integer type for use in such cases.  Addition,
subtraction, and comparison are the only operations supported.

Signed-off-by: Avi Kivity <avi@redhat.com>