Incorporate r1.140 and r1.141 of vmm.c:

Incorporate r1.140 and r1.141 of vmm.c:

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revision 1.341
date: 2023/04/26 09:39:56;  author: dv;  state: Exp;  lines: +3 -3;  commitid: 1XztmVbxa8OP2xYi;
Unbreak vmm on Intel. Mistakenly enabled CET on non-CET systems.

Found by anton@ & jsg@
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revision 1.340
date: 2023/04/24 16:53:57;  author: dv;  state: Exp;  lines: +11 -1;  commitid: q9qQ55f7h2gen8N4;
vmm(4): allow guests to enable and use supervisor IBT.

Why should hosts have all the fun? Conditionally unmask the cpuid
bits for IBT and allow r/w access to the supervisor CET msr.

Will need revisiting when we introduce usage of userland CET msr.