kvm-irqchip: introduce new API to support route change

kvm-irqchip: introduce new API to support route change

Paolo suggested adding the new API to support route changes [1]. We should invoke
kvm_irqchip_begin_route_changes() before changing the routes, increasing the
KVMRouteChange.changes if the routes are changed, and commit the changes at last.

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-11/msg02898.html

Signed-off-by: Longpeng <[email protected]>
Message-Id: <20220222141116[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>