doc: add how to exit from QEMU in samples

While trying out the hello_world sample built for QEMU, I was expecting
the sample app to exit and I'd return to a command prompt.  Nope.  You
need to exit QEMU manually, so add that step to the sample instructions.
Looking around, there are more uses of QEMU like this that could use
this added step after running the sample app.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>