BDRV_POLL_WHILE() does not support recursive AioContext locking.  It
only releases the AioContext lock once regardless of how many times the
caller has acquired it.  This results in a hang since the IOThread does
not make progress while the AioContext is still locked.

The following steps trigger the hang:

  $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \
                       -object iothread,id=iothread0 \
                       -device virtio-scsi-pci,iothread=iothread0 \
                       -drive if=none,id=drive0,file=test.img,format=raw \
                       -device scsi-hd,drive=drive0 \
                       -drive if=none,id=drive1,file=test.img,format=raw \
                       -device scsi-hd,drive=drive1
  $ qemu-system-x86_64 ...same options... \
                       -incoming tcp::1234
  (qemu) migrate tcp:127.0.0.1:1234
  ...hang...

Tested-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20171207201320.19284-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>