vcl svm: fix rx event loss

vcl svm: fix rx event loss

When vcl_epoll_wait_handle_mq handles rx events exceeding maxevents, VPP will not signal because cursize > 0, and the remaining rx events cannot be triggered because the eventfd event has been read. Therefore, we should dequeue all events until cursize = 0. And then handle msg up to maxevents with vcl_epoll_wait_handle_mq_event and those beyond with vcl_handle_mq_event.

Type: fix
Ticket: VPP-1873
Signed-off-by: hanlin <hanlin_wang@163.com>
Change-Id: I8a0c87cb41c837deb8284b40f668cc3c7d9d6e56
Signed-off-by: hanlin <hanlin_wang@163.com>
(cherry picked from commit d0e646f6892e9c85278c9538760a8940c86dcdbb)