x86/hvm: permit HVM_PARAM_IDENT_PT to be set more than once

x86/hvm: permit HVM_PARAM_IDENT_PT to be set more than once

There is no conceptual problem with setting this parameter more than once.
Checkpointed migration streams will typically set it once per checkpoint to
the same value.

The parameter is only actually needed on early-generation VT-x which lacked
the unrestricted guest capability, although it could plausibly be used on
newer VT-x with unusual execution control settings.  Short circuit the
expensive operations on non VT-x hardware.

The parameter itself must always be latched to avoid issues if the VM
eventually migrates to a host which needs to use the pagetable.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>