v2v: -i libvirt: Don't access remote -ic URIs during object creation.

Previously if you used `-i libvirt -ic [a remote URI]' then the remote
URI would be opened during object creation, which occurs during
command line parsing.  The practical effect was that virt-v2v could
appear to hang or give an error (eg if the remote URI is inaccessible)
before printing any log messages.  This could be very confusing for
the user.

Split out -i libvirtxml into a separate module, containing the libvirt
XML to source parsing code as a function called
Input_libvirtxml.parse_libvirt_xml.

Modify -i libvirt so it does all initialization and calls the helper
function during the #source method, and not at object creation time.