net/vhost: fix interrupt mode

net/vhost: fix interrupt mode

At .new_device() time, only the first vring pair is
now ready, other vrings are configured later.

Problem is that when application will setup and enable
interrupts, only the first queue pair Rx interrupt will
be enabled.

This patches fixes the issue by setting the number of
max interrupts to the number of Rx queues that will be
later initialized. Then, as soon as a Rx vring is ready
and interrupt enabled by the application, it removes the
corresponding uninitialized epoll event, and installs a
new one with the valid FD.

Fixes: 604052ae5395 ("net/vhost: support queue update")

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>