usb-host: skip open on pending postload bh

usb-host: skip open on pending postload bh

usb-host emulates a device unplug after live migration, because the
device state is unknown and unplug/replug makes sure the guest
re-initializes the device into a working state.  This can't be done in
post-load though, so post-load just schedules a bottom half which
executes after vmload is complete.

It can happen that the device autoscan timer hits the race window
between scheduling and running the bottom half, which in turn can
triggers an assert().

Fix that issue by just ignoring the usb_host_open() call in case the
bottom half didn't execute yet.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1572851
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20180503062932.17233-1-kraxel@redhat.com