pci_device_is_present() previously didn't work for VFs because it reads the
Vendor and Device ID, which are 0xffff for VFs, which looks like they
aren't present.  Check the PF instead.

Wei Gong reported that if virtio I/O is in progress when the driver is
unbound or "0" is written to /sys/.../sriov_numvfs, the virtio I/O
operation hangs, which may result in output like this:

  task:bash state:D stack:    0 pid: 1773 ppid:  1241 flags:0x00004002
  Call Trace:
   schedule+0x4f/0xc0
   blk_mq_freeze_queue_wait+0x69/0xa0
   blk_mq_freeze_queue+0x1b/0x20
   blk_cleanup_queue+0x3d/0xd0
   virtblk_remove+0x3c/0xb0 [virtio_blk]
   virtio_dev_remove+0x4b/0x80
   ...
   device_unregister+0x1b/0x60
   unregister_virtio_device+0x18/0x30
   virtio_pci_remove+0x41/0x80
   pci_device_remove+0x3e/0xb0

This happened because pci_device_is_present(VF) returned "false" in
virtio_pci_remove(), so it called virtio_break_device().  The broken vq
meant that vring_interrupt() skipped the vq.callback() that would have
completed the virtio I/O operation via virtblk_done().

[bhelgaas: commit log, simplify to always use pci_physfn(), add stable tag]
Link: https://lore.kernel.org/r/20221026060912.173250-1-mst@redhat.comReported-by: NWei Gong <gongwei833x@gmail.com>
Tested-by: NWei Gong <gongwei833x@gmail.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org