xen-netback: Minor refactoring of netback code

This patch contains a few bits of refactoring before introducing the grant
mapping changes:
- introducing xenvif_tx_pending_slots_available(), as this is used several
  times, and will be used more often
- rename the thread to vifX.Y-guest-rx, to signify it does RX work from the
  guest point of view

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc drivers/net/xen-netback/common.h
Simple merge

diff --cc drivers/net/xen-netback/interface.c
Simple merge

diff --cc drivers/net/xen-netback/netback.c
Simple merge