arm_gic_kvm: Disable live migration if not supported

arm_gic_kvm: Disable live migration if not supported

Currently, if the kernel does not have live migration API, the migration
will still be attempted, but vGIC save/restore functions will just not do
anything. This will result in a broken machine state.

This patch fixes the problem by adding migration blocker if kernel API is
not supported.

Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>