qemu: hotplug: wait for the tray to eject only for drives with a tray

qemu: hotplug: wait for the tray to eject only for drives with a tray

Use the detected tray presence flag to trigger the tray waiting code
only if the given storage device in qemu reports to have a tray.

This is necessary as the floppy device lost it's tray as of qemu commit:

commit abb3e55b5b718d6392441f56ba0729a62105ac56
Author: Max Reitz <mreitz@redhat.com>
Date:   Fri Jan 29 20:49:12 2016 +0100

    Revert "hw/block/fdc: Implement tray status"

(cherry picked from commit 72a7ff6b507bcf389cc493ac0ba07d32de266d6e)