hw/arm/virt: Use VIRT_GIC_VERSION_* enum values in create_gic()

hw/arm/virt: Use VIRT_GIC_VERSION_* enum values in create_gic()

Everywhere we need to check which GIC version we're using, we look at
vms->gic_version and use the VIRT_GIC_VERSION_* enum values, except
in create_gic(), which copies vms->gic_version into a local 'int'
variable and makes direct comparisons against values 2 and 3.

For consistency, change this function to check the GIC version
the same way we do elsewhere. This includes not implicitly relying
on the enumeration type values happening to match the integer
'revision' values the GIC device object wants.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220408141550.1271295-40-peter.maydell@linaro.org