Fix a problem properly saving URIs

As written by Martin Kletzander <mkletzan@redhat.com>:
Since commit 8eb55d782a2b9afacc7938694891cc6fad7b42a5, when you parse
and save an URI that has no server (or similar) part, two slashes
after the 'schema:' get lost.  It means 'uri:///noserver' is turned
into 'uri:/noserver'.

basically
   foo:///only/path

means a host of "" while

   foo:/only/path

means no host at all

  So the best fix IMHO is to fix the URI parser to record the first
case and an empty host string and the second case as a NULL host string

 I would not revert the initial patch, we should not 'invent' those
slash, but we should instead when parsing keep the information that
it's a host based path and that foo:/// means the presence of a host
but an empty one.

Once applied the resulting patch below, all cases seems to be saved
properly:

thinkpad:~/XML -> ./testURI uri:/noserver
uri:/noserver
thinkpad:~/XML -> ./testURI uri:///noserver
uri:///noserver
thinkpad:~/XML -> ./testURI uri://server/foo
uri://server/foo
thinkpad:~/XML -> ./testURI uri:/noserver/foo
uri:/noserver/foo
thinkpad:~/XML -> ./testURI uri:///
uri:///
thinkpad:~/XML -> ./testURI uri://
uri://
thinkpad:~/XML -> ./testURI uri:/
uri:/
thinkpad:~/XML ->

  If you revert the initial patch that last case fails

The problem is that I don't want to change the xmlURI structure to
minimize ABI breakage, so I could not extend the field. The natural
solution is to denote that uri:/// has an empty host by making
the uri server field an empty string which works very well but breaks
applications (like libvirt ;-) who blindly look at uri->server
not being NULL to try to reach it !
Simplest was to stick the port to -1 in that case, instead of 0
application don't bother looking at the port of there is no server
string, this makes the patch more complex than a 1 liner, but
is better for ABI.