If a pci address had a function number out of range, the error message
would be:

  Insufficient specification for PCI address

which is logged by virDevicePCIAddressParseXML() after
virDevicePCIAddressIsValid returns a failure.

This patch enhances virDevicePCIAddressIsValid() to optionally report
the error itself (since it is the place that decides which part of the
address is "invalid"), and uses that feature when calling from
virDevicePCIAddressParseXML(), so that the error will be more useful,
e.g.:

  Invalid PCI address function=0x8, must be <= 7

Previously, virDevicePCIAddressIsValid didn't check for the
theoretical limits of domain or bus, only for slot or function. While
adding log messages, we also correct that ommission. (The RNG for PCI
addresses already enforces this limit, which by the way means that we
can't add any negative tests for this - as far as I know our
domainschematest has no provisions for passing XML that is supposed to
fail).

Note that virDevicePCIAddressIsValid() can only check against the
absolute maximum attribute values for *any* possible PCI controller,
not for the actual maximums of the specific controller that this
device is attaching to; fortunately there is later more specific
validation for guest-side PCI addresses when building the set of
assigned PCI addresses. For host-side PCI addresses (e.g. for
<hostdev> and for network device pools), we rely on the error that
will be logged when it is found that the device doesn't actually
exist.

This resolves:

  https://bugzilla.redhat.com/show_bug.cgi?id=1004596