HCRX_GUEST_FLAGS gives random KVM hackers the impression that
they can stuff bits in this macro and unconditionally enable
features in the guest.

In general, this is wrong (we have been there with FEAT_MOPS,
and again with FEAT_TCRX).

Document that HCRX_EL2.SMPME is an exception rather than the rule,
and get rid of HCRX_GUEST_FLAGS.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20240625130042.259175-3-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>