Allow machines to configure the QEMU_VERSION that's exposed via hardware

QEMU exposes its version to the guest's hardware and in some cases that is wrong
(e.g. Windows prints messages about driver updates when you switch
the QEMU version).
There is a new field now on the struct QEmuMachine, hw_version, which may
contain the version that the specific machine should report. If that field is
set, then that machine will report that version to the guest.

Signed-off-by: Crístian Viana <vianac@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
