x86/cpuid: Hide VT-x/SVM from HVM-based control domains

x86/cpuid: Hide VT-x/SVM from HVM-based control domains

The VT-x/SVM features are hidden from PV dom0 by the pv_featureset[] upper
mask, but nothing thus far has prevented the features being visible in
HVM-based control domains (where there is no toolstack decision to hide the
features).

As a side effect of calling nestedhvm_enabled() earlier during domain
creation, it needs to cope with the params[] array not having been allocated.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>