conf: use virXMLFormatElement() in virDomainDeviceInfoFormat()
conf: use virXMLFormatElement() in virDomainDeviceInfoFormat()

In order to add zPCI child element for PCI address, we update
virDomainDeviceInfoFormat() to format device info by helper function
virXMLFormatElement(). Then we could simply format zPCI address into
child buffer later.

Signed-off-by: Yi Min Zhao <5bdf0032410856ea949e1abce934f6d55d9c7ba6@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <2648abdd55bfebf313fd324b464b047e6eea300e@linux.ibm.com>
Reviewed-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>

crobinso/libvirt,andreabolognani/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,eskultety/libvirt,olafhering/libvirt,andreabolognani/libvirt,jardasgit/libvirt,eskultety/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,olafhering/libvirt,eskultety/libvirt,andreabolognani/libvirt,crobinso/libvirt,libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,zippy2/libvirt,nertpinx/libvirt,zippy2/libvirt,jardasgit/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,olafhering/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,jardasgit/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jardasgit/libvirt