qdev: Fix assert in PCI address property when used by vfio-pci

Allow the PCIHostDeviceAddress structure to work as the host property
in vfio-pci when it has it's default value of all fields set to ~0. In
this form the property indicates a non-existant device but given the
field bit sizes gets asserted as excess (and invalid) precision
overflows the string buffer. The BDF of an invalid device
"FFFF:FF:FF.F" is returned instead.

Signed-off-by: Daniel Oram <daniel.oram@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <71f06765c4ba16dcd71cbf78e877619948f04ed9.1478777270.git.daniel.oram@gmail.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
