hw/arm/virt: Reject gic-version=host for non-KVM

If you try to gic-version=host with TCG on a KVM aarch64 host,
qemu segfaults, since host requires KVM APIs.

Explicitly reject gic-version=host if KVM is not enabled

https://bugzilla.redhat.com/show_bug.cgi?id=1339977
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: b1b3b0dd143b7995a7f4062966b80a2cf3e3c71e.1464273085.git.crobinso@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
