KVM: VMX: Fix interrupt exit condition during emulation

Checking EFLAGS.IF is incorrect as we might be in interrupt shadow.  If
that is the case, the main loop will notice that and not inject the interrupt,
causing an endless loop.

Fix by using vmx_interrupt_allowed() to check if we can inject an interrupt
instead.

Signed-off-by: Avi Kivity <avi@redhat.com>