Increase QEMU_MAX_MIGRATION_TIME to avoid incomplete tests
    
    We see tests periodically come up incomplete with this:
    "Migrate to file failed, it has been running for more than 240 seconds"
    Turns out there's a setting for this, so...let's set it. I don't
    think there's really a bug here, just when our workers are busy,
    these migrations can take longer than expected.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>