atomic: fix position of volatile qualifier

What needs to be volatile is not the pointer, but the pointed-to
value!

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 2cbcfb28)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>