kvm tool: serial: Fix interrupt handling

The interrupt injection of the serial emulation is completely
broken. It's just doing random toggling of the interrupt line, which
can lead to complete console hangs.

The real hardware asserts the interrupt line when a condition
(RX/TX/Status) is met and the corresponding interrupt is enabled in
the IER. It's deasserted when the condition is cleared or the
corresponding interrupt is disabled in the IER.

So the correct emulation just needs to check after each state change
in the LSR or the IER which bits in the IIR need to be set and update
the interrupt line accordingly. To avoid setting the same state over
and over keep an internal state of the last set interrupt line state
and only update via the kvm ioctl when the new state differs.

Rename serial8250__inject_interrupts() to serial8250__update_consoles()
which reflects what the function really is about.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Pekka Enberg <penberg@kernel.org>