Qemu fixes, and a bonus

A number of people have reported problems with qemu and DragonFly, both running locally and on a host.  It turns out to be a problem with the getcontext(), setcontext(), and swapcontext() functions, but Matthew Dillon fixed it in a way that doesn’t affect performance very much.

That apparently wasn’t good enough, so he added _quick versions of those same functions, so it became not just a fix, but an improvement.

In related qemu news: qemu-devel can use vknetd similar to a vkernel, now.