blockdev: Allow more options for BB-less BDS tree

Most of the options which blockdev_init() parses for both the
BlockBackend and the root BDS are valid for just the root BDS as well
(e.g. read-only). This patch allows specifying these options even if not
creating a BlockBackend.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
