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

This reverts commit e6976213.
It is causing problems dependeing on what version of QEMU is
used, with the SDK QEMU the terminal has to be 'reset' after
running (the opposite behavior of the Nios 2 build).

Change-Id: I6756eb0cf8edf4faa36f3a5e1cf0e456f146e1d8
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>