qemuboot-x86: Switch to IvyBridge and q35 instead of pc

Move from 1996 to 2009 by swapping machine 'pc' for 'q35'.

Also move to a CPU which is SMP capable and doesn't have tsc bugs. IvyBridge
matches what we're using on the autobuilder.

The intent here is to try and improve on some of the intermittent autobuilder
issues we're seeing. I'm told that nobody else runs with config this old
and it could well be contributing to our issues. Having reliable testing
is key to the project and justifies updating this IMO.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>