x86/hvm: Obsolete the use of HVM_PARAM_NESTEDHVM

x86/hvm: Obsolete the use of HVM_PARAM_NESTEDHVM

With XEN_DOMCTL_CDF_nested_virt now passed properly to domain_create(),
reimplement nestedhvm_enabled() to use the property which is fixed for the
lifetime of the domain.

This makes the call to nestedhvm_vcpu_initialise() from hvm_vcpu_initialise()
no longer dead.  It became logically dead with the Xend => XL transition, as
they initialise HVM_PARAM_NESTEDHVM in opposite orders with respect to
XEN_DOMCTL_max_vcpus.

There is one opencoded user of nestedhvm_enabled() in HVM_PARAM_ALTP2M's
safety check.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wl@xen.org>