exec: Stop using memory after free

memory_region_unref(mr) can free memory.

For example I got:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f43280d4700 (LWP 4462)]
0x00007f43323283c0 in phys_section_destroy (mr=0x7f43259468b0)
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1023
1023        if (mr->subpage) {
(gdb) bt
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1023
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1034
    at /home/don/xen/tools/qemu-xen-dir/exec.c:2205
(gdb) p mr
$1 = (MemoryRegion *) 0x7f43259468b0

And this change prevents this.

Signed-off-by: Don Slutz <Don.Slutz@Gmail.com>
Message-Id: <1448921464-21845-1-git-send-email-Don.Slutz@Gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
