qemu: pass -serial stdio instead of -serial mon:stdio

You can now exit out of QEMU using Ctrl-C, and this resolves
some issues where the terminal could get messed up if QEMU
terminates abnormally.

Change-Id: I94ca66333bf9035e3627f28bbd2c152cf981a13f
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>