pSeries guests will soon be allowed to have multiple
PHBs (pci-root controllers), which of course means that
all but one of them will have a non-zero index; hence,
we'll need to relax the current check.

However, right now the check is performed in the conf
module, which is generic rather than tied to the QEMU
driver, and where we don't have information such as the
guest machine type available.

To make this change of behavior possible down the line,
we need to move the check from the XML parser to the
drivers. Luckily, only QEMU and bhyve are using PCI
controllers, so this doesn't result in much duplication.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NLaine Stump <laine@laine.org>