qemu: replace multiple strcmps with a switch on an enum

qemu: replace multiple strcmps with a switch on an enum

I'm not sure why this code was written to compare the strings that it
had just retrieved from an enum->string conversion, rather than just
look at the original enum values, but this yields the same results,
and is much more efficient (especially as you add more devices).

This is a prerequisite for patches to resolve:

   https://bugzilla.redhat.com/show_bug.cgi?id=1003983