hw/arm/virt: kvm: Restructure finalize_gic_version()

hw/arm/virt: kvm: Restructure finalize_gic_version()

Restructure the finalize_gic_version with switch cases and
clearly separate the following cases:

- KVM mode / in-kernel irqchip
- KVM mode / userspace irqchip
- TCG mode

In KVM mode / in-kernel irqchip , we explictly check whether
the chosen version is supported by the host. If the end-user
explicitly sets v2/v3 and this is not supported by the host,
then the user gets an explicit error message. Note that for
old kernels where the CREATE_DEVICE ioctl doesn't exist then
we will now fail if the user specifically asked for gicv2,
where previously we (probably) would have succeeded.

In KVM mode / userspace irqchip we immediatly output an error
in case the end-user explicitly selected v3. Also we warn the
end-user about the unexpected usage of gic-version=host in
that case as only userspace GICv2 is supported.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-id: 20200311131618.7187-6-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>