In an upcoming patch, virDomainNetDef will acquire a
virDomainHostdevDef, and the <interface> XML will take on some of the
elements of a <hostdev>. To avoid duplicating the code for parsing and
formatting the <source> element (which will be nearly identical in
these two cases), this patch factors those parts out of the
HostdevDef's parse and format functions, and puts them into separate
helper functions that are now called by the HostdevDef
parser/formatter, and will soon be called by the NetDef
parser/formatter.

One change in behavior - previously virDomainHostdevDefParseXML() had
diverged from current common coding practice by logging an error and
failing if it found any subelements of <hostdev> other than those it
understood (standard libvirt practice is to ignore/discard unknown
elements and attributes during parse). The new helper function ignores
unknown elements, and thus so does the new
virDomainHostdevDefParseXML.