[Libguestfs] [PATCH v3] lib: libvirt: Convert all drive socket parameters to an absolute path (RHBZ#1588451).

https://bugzilla.redhat.com/show_bug.cgi?id=1588451 v2 was here: https://www.redhat.com/archives/libguestfs/2018-June/msg00067.html This is a greatly simplified version, which just changes the libvirt backend to make the path absolute. None of the tests need to be adjusted. Rich.