Xen: Improve parsing of PCI addresses in config converter

Xen: Improve parsing of PCI addresses in config converter

There was a report on libvirt-users [1] about the domxml to/from
native converter in the Xen driver not handling PCI addresses
without a domain specification. This patch improves parsing of PCI
addresses in the converter and allows PCI addresses with only
bb:ss.f. xl.cfg(5) also allows either the dddd:bb:ss.f or bb:ss.f
format. A test has been added to check the conversion from xl.cfg
to domXML.

[1] https://www.redhat.com/archives/libvirt-users/2020-August/msg00040.html

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>