Don't use "hz" in identifiers to make AIX happy.

malc found AIX headers leak "hz" and so it can't be used there.  Change
the occurences in hw/.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5709 c046a42c-6fe2-441c-8c8c-71466251a162
