configs/qemu: enable SMP for x86 & x86_64

This is the 21st century, so enable SMP for them.

[Peter: add note to readme.txt about how emulate a SMP system in QEMU]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>