[v2] qemu: Retry JSON monitor cont cmd on MigrationExpected error

[v2] qemu: Retry JSON monitor cont cmd on MigrationExpected error

When restoring a saved qemu instance via JSON monitor, the vm is
left in a paused state.  Turns out the 'cont' cmd was failing with
"MigrationExpected" error class and "An incoming migration is
expected before this command can be executed" error description
due to migration (restore) not yet complete.

Detect if 'cont' cmd fails with "MigrationExpecte" error class and
retry 'cont' cmd.

V2: Fix potential double-free noted by Laine Stump