hw/arm/virt: Update error message for bad gic-version option

As we added different valid gic-version option settings,
we forgot to update the hint in the error message produced
when the user specifies an invalid value:

$ qemu-system-aarch64 -M virt,help | grep gic-version
  gic-version=<string>   - Set GIC version. Valid values are 2, 3, 4, host and max
$ qemu-system-aarch64 -M virt,gic-version=bang
qemu-system-aarch64: Invalid gic-version value
Valid values are 3, 2, host, max.

Update the error string to match the one we use in the help text
for the option.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20260219110228.3804482-1-peter.maydell@linaro.org