KVM: SVM: Handle #MCs in guest outside of fastpath

Handle Machine Checks (#MC) that happen in the guest (by forwarding them
to the host) outside of KVM's fastpath so that as much host state as
possible is re-loaded before invoking the kernel's #MC handler.  The only
requirement is that KVM invokes the #MC handler before enabling IRQs (and
even that could _probably_ be relaxed to handling #MCs before enabling
preemption).

Waiting to handle #MCs until "more" host state is loaded hardens KVM
against flaws in the #MC handler, which has historically been quite
brittle. E.g. prior to commit 5567d11c21a1 ("x86/mce: Send #MC singal from
task work"), the #MC code could trigger a schedule() with IRQs and
preemption disabled.  That led to a KVM hack-a-fix in commit 1811d979c716
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Note, except for #MCs on VM-Enter, VMX already handles #MCs outside of the
fastpath.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20251118222328.2265758-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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