QEMU is not blocking NMIs on entry to SMM.  Implementing this has to
cover a few corner cases, because:

- NMIs can then be enabled by an IRET instruction and there
is no mechanism to _set_ the "NMIs masked" flag on exit from SMM:
"A special case can occur if an SMI handler nests inside an NMI handler
and then another NMI occurs. [...] When the processor enters SMM while
executing an NMI handler, the processor saves the SMRAM state save map
but does not save the attribute to keep NMI interrupts disabled.

- However, there is some hidden state, because "If NMIs were blocked
before the SMI occurred [and no IRET is executed while in SMM], they
are blocked after execution of RSM."  This is represented by the new
HF2_SMM_INSIDE_NMI_MASK bit.  If it is zero, NMIs are _unblocked_
on exit from RSM.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>