hw/arm/virt: Fix property "gic-version" error handling

virt_set_gic_version() calls exit(1) when passed an invalid property
value.  Property setters are not supposed to do that.  Screwed up in
commit b92ad39.  Harmless, because the property belongs to a machine.
Set an error object instead.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
