If XSetSelectionOwner() is invoked during the time
we are waiting for the requested clipboard targets,
the targets we eventually receive are no longer valid.

To solve this, ignore the same count of target notifications
as we expected at the time we received grab from the client.

Otherwise we end up in a situation when vdagent holds
the clipboard grab in the guest but cannot provide data to the
apps that request it - this can be observed in the log:

    clipboard: received selection request event for target *, while not owning client clipboard

Signed-off-by: Jakub Janků <jjanku@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>