xen_common: Change xenParsePCIList to use virConfGetValueStringList
xen_common: Change xenParsePCIList to use virConfGetValueStringList

The `if(!list || list->type != VIR_CONF_LIST)` check couldn't be
written in a 100% similar way. Instead, we're just checking whether
`virConfGetValueStringList() <= 0` and creating a new function to:
- return -1 in case virConfGetValueStringList fails either due to some
  allocation failure or when traversing the list;
- resetting the last error and return 0 otherwise;

Taking this approach we can have the behaviour with the new code as
close as possible to the old one.

Signed-off-by: Fabiano Fidêncio <2931bae082ef9bc04523ba759c03a4778316d48f@redhat.com>
Reviewed-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,libvirt/libvirt,jardasgit/libvirt,eskultety/libvirt,zippy2/libvirt,crobinso/libvirt,zippy2/libvirt,andreabolognani/libvirt,nertpinx/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,andreabolognani/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,eskultety/libvirt,andreabolognani/libvirt,eskultety/libvirt,andreabolognani/libvirt,crobinso/libvirt,olafhering/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,nertpinx/libvirt