vfio/pci: Fix handling of pci use accessor return codes

The pci user accessors return negative errno's on error.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
[aw: drop Fixes tag, pcibios_err_to_errno() behaves correctly for -errno]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>