kvm,8250: Fix "too much work for irq4" problems

This patch fixes the "too much work for irq4" problems caused by the fact that
we never set the UART_IIR_NO_INT flag in the emulation layer. This makes the
interrupt handler in the guest kernel process as much as possible which
triggers a warning.

Signed-off-by: Pekka Enberg <penberg@kernel.org>