QEMU set irq0override in fw_cfg

Hi,

After discussing the issue with Avi, Gleb and a couple others on irq,
we came to the conclusion that it is preferred to have QEMU request
features from the BIOS, rather than notifying the BIOS that it is
running on QEMU or KVM. This way memory ranges can change etc. and
an older BIOS will continue to work on newer QEMU if it receives the
info as a fw_cfg value.

This one also matches what qemu-kvm does for irq0override, except I
haven't made it configurable. I leave that as an exercise for whoever
would be interested in switching off irq0override.

Thanks,
Jes

Set irq0 override in fw_cfg, informing the BIOS that QEMU expects
override on irq0. This matches qemu-kvm, and will help sharing a
single BIOS binary.

Signed-off-by: Jes Sorensen <jes@sgi.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
