upgrade: force upgrade to v2 before upgrading to v3

During the upgrade of a domain to version 3 vdsm reallocates the
metadata slots that are higher than 1947 (given a leases LV of 2Gb)
in order to use the same offsets for the volume leases (BZ#882276
and git commit hash 2ba76e3).
This has no effect when the domain is version 0 since the metadata
slots offsets are fixed (the first physical extent of the LV) and
they can't be reallocated. In such case the domain must be upgraded
to version 2 first.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=893184
Change-Id: I2bd424ad29e76d1368ff2959bb8fe45afc595cdb
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10792
Reviewed-by: Ayal Baron <abaron@redhat.com>
Tested-by: Haim Ateya <hateya@redhat.com>