qemu-system-alpha crashes during migration
Description of problem:
QEMU crashes (aborts) when trying to migrate with qemu-system-alpha.

```
qemu-system-alpha: migration/ram.c:874: pss_find_next_dirty: Assertion `pss->host_page_end' failed.
```
Steps to reproduce:
1. Run `./qemu-system-alpha -incoming tcp:0:1234` in one terminal
2. Run `./qemu-system-alpha -monitor stdio` in another terminal
3. Type `migrate tcp:0:1234` in the HMP monitor