vfio/pci: Allow VPD short read

The size of the VPD area is not necessarily 4-byte aligned, so a
pci_vpd_read() might return less than 4 bytes.  Zero our buffer and
accept anything other than an error.  Intel X710 NICs exercise this.

Fixes: 4e1a635 ("vfio/pci: Use kernel VPD access functions")
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_config.c

@@ -749,7 +749,8 @@ static int vfio_vpd_config_write(struct vfio_pci_device *vdev, int pos,
 		if (pci_write_vpd(pdev, addr & ~PCI_VPD_ADDR_F, 4, &data) != 4)
 			return count;
 	} else {
-		if (pci_read_vpd(pdev, addr, 4, &data) != 4)
+		data = 0;
+		if (pci_read_vpd(pdev, addr, 4, &data) < 0)
 			return count;
 		*pdata = cpu_to_le32(data);
 	}