util: support device stats collection for <interface type='hostdev'>

util: support device stats collection for <interface type='hostdev'>

libvirt can retrieve traffic stats for emulated interfaces that are
backed by tap or macvtap devices, but this information wasn't
available for hostdev interfaces (those that are implemented by
assigning an SR-IOV VF device to a guest using vfio):

  #virsh domifstat instance --interface=52:54:00:2d:b2:35
  error: Failed to get interface stats instance 52:54:00:2d:b2:35
  error: internal error: Interface name not provided

For some SR-IOV VF devices this information is available via the
netlink VFINFO_LIST request/response, and that is what this patch uses
to implement stats retrieval for VF. Not that this is dependent on
support in the PF driver - for example, the Mellanox ConnectX-4 Lx
(mlx5) driver reports usable stats, while Intel 82599 (ixgbe) and
82576 (igb) just report all stats as 0.  (this is the same result as
"ip -s link show").

Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Laine Stump <laine@redhat.com>