KVM: x86: clear SMM flags before loading state while leaving SMM

KVM: x86: clear SMM flags before loading state while leaving SMM

RSM emulation is currently broken on VMX when the interrupted guest has
CR4.VMXE=1.  Stop dancing around the issue of HF_SMM_MASK being set when
loading SMSTATE into architectural state, e.g. by toggling it for
problematic flows, and simply clear HF_SMM_MASK prior to loading
architectural state (from SMRAM save state area).

Reported-by: Jon Doron <[email protected]>
Cc: Jim Mattson <[email protected]>
Cc: Liran Alon <[email protected]>
Cc: Vitaly Kuznetsov <[email protected]>
Fixes: 5bea5123cbf0 ("KVM: VMX: check nested state and CR4.VMXE against SMM")
Signed-off-by: Sean Christopherson <[email protected]>
Tested-by: Vitaly Kuznetsov <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>