CA-143944: Allow suspend when PV drivers are present (as before)

A few commits ago, we made a change to allow suspending based on the presence of
the "feature-suspend" key. Unfortunately, the Windows guest agent only writes
this key on startup and not on resume, so that xapi refuses any suspend request
after a resume (and probably after migration). As a workaround, we re-add the
old-style check, where the presence of the PV-drivers implies that all features
are enabled.

As #1895, but for master.