launch: libvirt: Refactor seclabel code.

This is just code motion.
libvirt: Add debugging when opening libvirt handle.

When debugging is enabled, this produces output like below.  This is
useful when diagnosing what URI libguestfs is using.

libguestfs: opening libvirt handle: URI = NULL, auth = virConnectAuthPtrDefault, flags = 0
libguestfs: successfully opened libvirt handle: conn = 0xb05580
launch: libvirt: Add 'is_root' flag to the params struct.

This is just code motion, but it allows us to read this flag inside
the 'construct_libvirt_xml_seclabel' function in future (as a possible
way to fix RHBZ#890291).