Fix per-vg checks in instance disk conversion
Fix per-vg checks in instance disk conversion

Since we now have just two possibilities (plaint to drbd or drbd to
plain), we can assume (+an assert) that we come from LVM and access
directly logical_id[0] for the VG on which it is stored.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Guido Trotter <f62ca4d5358bdaeef0e50e57f9f7e231de63727e@google.com>

dimara/ganeti,leshchevds/ganeti,apyrgio/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,grnet/snf-ganeti,andir/ganeti,bitemyapp/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,apyrgio/snf-ganeti,andir/ganeti,grnet/snf-ganeti,yiannist/ganeti,ganeti/ganeti,apyrgio/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,mbakke/ganeti,mbakke/ganeti,onponomarev/ganeti,bitemyapp/ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,dimara/ganeti