kvm,serial: Implement 16550A FIFO support

This patch implements 16550A FIFO support to the serial console emulation
layer. There's still a bug lurking somewhere which the hypervisor in some busy
loop taking up 100% of CPU. However, this patch is a definite improvement over
the previous hacks.

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