PCI: Simplify IOV implementation and fix reference count races

Trivial changes to IOV:

  1) use new PCI interfaces to simplify IOV implementation
  2) fix some reference count related race windows

[bhelgaas: fix virtfn_add() add bus/alloc dev error paths]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Donald Dutile <ddutile@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Ram Pai <linuxram@us.ibm.com>