nfs: move nfs_set_events out of the while loops

nfs_set_events only needs to be called once before entering the
while loop; afterwards, nfs_process_read and nfs_process_write
take care of it.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1477565348-5458-9-git-send-email-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
