ide: fix halted IO segfault at reset

If one attempts to perform a system_reset after a failed IO request
that causes the VM to enter a paused state, QEMU will segfault trying
to free up the pending IO requests.

These requests have already been completed and freed, though, so all
we need to do is NULL them before we enter the paused state.

Existing AHCI tests verify that halted requests are still resumed
successfully after a STOP event.

Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1469635201-11918-2-git-send-email-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>