qemu: improve error reporting during PCI address validation

This patch addresses two concerns with the error reporting when an
incompatible PCI address is specified for a device:

1) It wasn't always apparent which device had the problem. With this
patch applied, any error about an incompatible address will always
contain the full address as given in the config, so it will be easier
to determine which device's config aused the problem.

2) In some cases when the problem came from bad config, the error
message was erroneously classified as VIR_ERR_INTERNAL_ERROR. With
this patch applied, the same error message will be changed to indicate
either "internal" or "xml" error depending on whether the address came
from the config, or was automatically generated by libvirt.

Note that in the case of "internal" (due to bad auto-generation)
errors, the PCI address won't be of much use in finding the location
in config to change (because it was automatically generated). Of
course that makes perfect sense, but still the address could provide a
clue about a bug in libvirt attempting to use a type of pci bus that
doesn't have its flags set correctly (or something similar). In other
words, it's not perfect, but it is definitely better.