Fix disk adoption interaction with ipolicy checks

In Ganeti 2.6, disk adoption is broken due to the ipolicy checks being
done before we read volume size from remote nodes. We fix this by
simply moving these checks to after the disk adoption code which
updates the disk size; it's not that nice that we fail a (almost)
config-level check after we've reserved the LVs, etc., but we need to
do so in order to validate the ipolicy correctly.

Tested:

- normal instance creation
- creation via adoption with good size (pass)
- creation via adoption with wrong LV size (fail as expected)
- QA in progress

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>