If a VM-exit happens during an NMI injection then clear the "NMI Blocking" bit
in the Guest Interruptibility-state VMCS field.

If we fail to do this then a subsequent VM-entry will fail because it is an
error to inject an NMI into the guest while "NMI Blocking" is turned on. This
is described in "Checks on Guest Non-Register State" in the Intel SDM.

Submitted by: David Reed (david.reed@tidalscale.com)