Comment 9 for bug 1536487

> Especially I found that the "created on Trusty, migrated to xenial
> (works), but later migrated back to trusty (fails)" seems affected by
> it as well.

The first migration of the t->x->t sequence does not really matter (if
anything it could introduce _more_ bugs!), so if x->t is not supported
then neither is t->x->t.

The upstream QEMU project doesn't have the manpower to test and support
backwards migration. We try not to break it, and in this case there
was an easy fix and we suggest that Canonical backports it. However,
in general it's not guaranteed to work.

The fix is commit e6915b5f3a874a467a9a65f7ec1d6ef8d251a51a.