hw/arm/virt: Support -machine gic-version=max

Add support for passing 'max' to -machine gic-version. By analogy
with the -cpu max option, this picks the "best available" GIC version
whether you're using KVM or TCG, so it behaves like 'host' when
using KVM, and gives you GICv3 when using TCG.

Also like '-cpu host', using -machine gic-version=max' means there
is no guarantee of migration compatibility between QEMU versions;
in future 'max' might mean '4'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180308130626.12393-7-peter.maydell@linaro.org
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>