Loading migration of VM in debug state fails (with potential solution)
Description of problem:
```
qemu-system-x86_64: invalid runstate transition: 'inmigrate' -> 'debug'
Aborted (core dumped)
```
Steps to reproduce:
1. Start VM with gdbstub
2. Pause VM via gdbstub
3. Save migration snapshot via HMP: `migrate "exec: gzip -c > foo.gz"`
4. Start new QEMU instance from snapshot by adding these args to whatever you used to launch QEMU: `-incoming "exec: gzip -c -d foo.gz"`
Additional information:
This can be fixed by adding `{ RUN_STATE_INMIGRATE, RUN_STATE_DEBUG },` to `runstate_transitions_def` in `softmmu/runstate.c`. It's not clear if there are any negative ramifications of this, but it seems to work for me?