KVM: x86: unify handling of interrupt window

The interrupt window is currently checked twice, once in vmx.c/svm.c and
once in dm_request_for_irq_injection.  The only difference is the extra
check for kvm_arch_interrupt_allowed in dm_request_for_irq_injection,
and the different return value (EINTR/KVM_EXIT_INTR for vmx.c/svm.c vs.
0/KVM_EXIT_IRQ_WINDOW_OPEN for dm_request_for_irq_injection).

However, dm_request_for_irq_injection is basically dead code!  Revive it
by removing the checks in vmx.c and svm.c's vmexit handlers, and
fixing the returned values for the dm_request_for_irq_injection case.

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