throttle: Remove block from group on hot-unplug

When a block device that is part of a throttle group is hot-unplugged,
we forgot to remove it from the throttle group. This leaves stale
memory around, and causes an easily reproducible crash:

$ ./x86_64-softmmu/qemu-system-x86_64 -nodefaults -nographic -qmp stdio \
-device virtio-scsi-pci,bus=pci.0 -drive \
id=drive_image2,if=none,format=raw,file=file2,bps=512000,iops=100,group=foo \
-device scsi-hd,id=image2,drive=drive_image2 -drive \
id=drive_image3,if=none,format=raw,file=file3,bps=512000,iops=100,group=foo \
-device scsi-hd,id=image3,drive=drive_image3
{'execute':'qmp_capabilities'}
{'execute':'device_del','arguments':{'id':'image3'}}
{'execute':'system_reset'}

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1428810

Suggested-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 20170406190847.29347-1-eblake@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
