usb-storage can't handle requests in one go as the data transfer can be
splitted into lots of usb packets.  Because of that there can be
normal in-flight requests at savevm time and we need to handle that.
With other scsi hba's this happens only in case i/o is stopped due to
errors and there are pending requests which need to be restarted
(req->retry = true).

So, first we need to save req->retry and then handle the req->retry =
false case.  Write requests are handled fine already.  For read requests
we have to save the buffer as we will not restart the request (and thus
not refill the buffer) on the target host.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>