xenapi: make early failures in resize less dissruptive

Compleation of work on bug 1155066

Falling back on the instance actions to report resize failures.
Rather than full rollback with resize, make user errors quick
to recover from without and admin's assistence.
In particular, when you request a resize to a size that is too
small given your current disk usage.

Change-Id: I5ef525ad770d1a13999e4010c48d8c22a2a6010d