KVM: VMX: Do not trap VMFUNC instructions for L1 guests.

KVM: VMX: Do not trap VMFUNC instructions for L1 guests.

Explicitly disable VMFUNC in vmcs01 to document that KVM doesn't support
any VM-Functions for L1.  WARN in the dedicated VMFUNC handler if an exit
occurs while L1 is active, but keep the existing handlers as fallbacks to
avoid killing the VM as an unexpected VMFUNC VM-Exit isn't fatal

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Link: https://lore.kernel.org/r/20221109075413.1405803-2-yu.c.zhang@linux.intel.com
[sean: don't kill the VM on an unexpected VMFUNC from L1, reword changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>