Prior to this, <address type='pci'/> wasn't allowed when parsing
(domain+bus+slot+function needed to be a "valid" PCI address, meaning
that at least one of domain/bus/slot had to be non-0), the RNG
required bus to be specified, and if type was set to PCI when
formatting, domain+bus+slot+function would always be output.

This makes all the address attributes optional during parse and RNG
validation, and suppresses domain+bus+slot+function if domain+bus+slot
are all 0 (NB: if d+b+s are all 0, any value for function is
nonsensical as that will never happen in the real world, and after
the next patch we will always assign a real working address to any
empty PCI address before it is ever output to anywhere).

Note that explicitly setting all attributes to 0 is equivalent to
setting none of them, which is okay, since 0000:00:00 is reserved in
any PCI bus setup, and can't be used anyway.