In virsh we quite often get the domain XML just to initialize the XPath
parser so that we can extract information.
Add helpers which will simplify this by wrapping the getting of the XML
and parsing it along with error reporting.
Additionally a second helper also gets the domain object from the
parameters and releases it so that functions which need the XML as only
source of data can be simplified further.
virDomainFree has it's quirks (does not like NULL pointers, resets
libvirt errors). Replace it by a virsh helper which will allow us to
centrally fix issues with it.
The syntax-check rule will prohibit new uses of virDomainFree.
Move virshLookupDomainBy, virshCommandOptDomainBy and
virshCommandOptDomainBy to the helper file. Additionally turn the
virshCommandOptDomainBy macro into a function.
