block: freeze the backing chain earlier in stream_start()

block: freeze the backing chain earlier in stream_start()

Commit 6585493369819a48d34a86d57ec6b97cb5cd9bc0 added code to freeze
the backing chain from 'top' to 'base' for the duration of the
block-stream job.

The problem is that the freezing happens too late in stream_start():
during the bdrv_reopen_set_read_only() call earlier in that function
another job can jump in and remove the base image. If that happens we
have an invalid chain and QEMU crashes.

This patch puts the bdrv_freeze_backing_chain() call at the beginning
of the function.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>