[Libguestfs] [PATCH] Introduce a wrapper around xmlParseURI.

An alternate solution to: https://www.redhat.com/archives/libguestfs/2017-December/msg00035.html "[PATCH] v2v: -i vmx: Allow ssh URLs to use spaces." is to classify all URLs processed by libguestfs as either ordinary URLs or the special non-standard URLs that we use for things like ‘virt-v2v -i vmx’ and ‘guestfish --add’. For the non-standard URLs, provide a wrapper around xmlParseURI which turns spaces into %20, and other things. This patch reveals that there are bugs in the qemu ssh driver (or libssh2). It cannot handle spaces in the file path or in fact any non-ASCII character. Rich.