KVM: nSVM: Report interrupts as allowed when in L2 and exit-on-interrupt is set

Report interrupts as allowed when the vCPU is in L2 and L2 is being run with
exit-on-interrupts enabled and EFLAGS.IF=1 (either on the host or on the guest
according to VINTR).  Interrupts are always unblocked from L1's perspective
in this case.

While moving nested_exit_on_intr to svm.h, use INTERCEPT_INTR properly instead
of assuming it's zero (which it is of course).

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