qemusecuritymock: Fix bit arithmetic

One of the functions of this mock is that it spoofs chown() and
stat() calls. But it is doing so in a clever way: it stores the
new owner on chown() and reports it on subsequent stat(). This is
done by using a 32bit unsigned integer where one half is used to
store uid the other is for gid. Later, when stat() is called the
integer is fetched and split into halves again. Well, my bit
operation skills are poor and the code I've written does not do
that properly.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemusecuritymock.c

@@ -226,7 +226,7 @@ int virFileRemoveXAttr(const char *path,
                 sb->st_gid = DEFAULT_GID; \
             } else { \
                 /* Known path. Set values passed to chown() earlier */ \
-                sb->st_uid = *val % 16; \
+                sb->st_uid = *val & 0xffff; \
                 sb->st_gid = *val >> 16; \
             } \
 \