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: 4e1a6355

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