block: Add PreallocMode to BD.bdrv_truncate()

Add a PreallocMode parameter to the bdrv_truncate() function implemented
by each block driver. Currently, we always pass PREALLOC_MODE_OFF and no
driver accepts anything else.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170613202107.10125-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
14 files changed