tools/libs/foreignmemory: provide xenforeignmemory_unmap.

tools/libs/foreignmemory: provide xenforeignmemory_unmap.

And require it be used instead of direct munmap.

This will allow e.g. Valgrind hooks to help track incorrect use of
foreign mappings.

Switch all uses of xenforeignmemory_map to use
xenforeignmemory_unmap, not that foreign mappings via the libxc compat
xc_map_foreign_* interface will not take advantage of this and will
need converting.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>