xen-netback: enable skip_guestrx_thread by default

xen-netback: enable skip_guestrx_thread by default

Given that there's almost no grant ops being done, there's no need to
amortize grant hypercall cost through the guestrx kthread. This option
then skips it enterily (only with staging grants) and gets to remove the
contention on queue->wq and queue->rx_lock.

Exadata chatty select tests show an improvement of 14% as depicted in
the bug.

Orabug: 27125766
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>