block: Attach non-qdev devices as well

block: Attach non-qdev devices as well

For now, this just protects against programming errors like having the
same drive back multiple non-qdev devices, or untimely bdrv_delete().
Later commits will add other interesting uses.

While there, rename BlockDriverState member peer to dev, bdrv_attach()
to bdrv_attach_dev(), bdrv_detach() to bdrv_detach_dev(), and
bdrv_get_attached() to bdrv_get_attached_dev().

Signed-off-by: Markus Armbruster <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>