vfio/pci: replace fixed string limit by g_strdup_printf

A trivial change to remove string limit by using g_strdup_printf

Tested-by: Neo Jia <cjia@nvidia.com>
Signed-off-by: Neo Jia <cjia@nvidia.com>
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
