hw/pass-through.c: workaround for old libpci

hw/pass-through.c: workaround for old libpci

Old versions of libpci (including the ones on the automatic tests
which control Xen staging propagation) do not define
PCI_LIB_VERSION or the PCI_ERR_{UNCOR_MASK,...} constants.

This means that change 8c771eb6294afc5b3754a9e3de51568d4e5986c2 breaks
the build.  In this changeset I apply what is intended to be a
workaround for this problem but it may not be completely correct; this
is therefore perhaps an interim fix.

The potential problem is that the save/restore of some PCI passthrough
error handling registers (across suspend/resume) may not work properly
with old versions of libpci.  However non-passthrough and non-suspect
use cases should now be fine.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>