x86/spec-ctrl: Make VERW flushing runtime conditional

x86/spec-ctrl: Make VERW flushing runtime conditional

Currently, VERW flushing to mitigate MDS is boot time conditional per domain
type.  However, to provide mitigations for DRPW (CVE-2022-21166), we need to
conditionally use VERW based on the trustworthiness of the guest, and the
devices passed through.

Remove the PV/HVM alternatives and instead issue a VERW on the return-to-guest
path depending on the SCF_verw bit in cpuinfo spec_ctrl_flags.

Introduce spec_ctrl_init_domain() and d->arch.verw to calculate the VERW
disposition at domain creation time, and context switch the SCF_verw bit.

For now, VERW flushing is used and controlled exactly as before, but later
patches will add per-domain cases too.

No change in behaviour.

This is part of XSA-404.

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