block: include original filename when reporting invalid URIs

Consider passing a JSON based block driver to "qemu-img commit"

$ qemu-img commit 'json:{"driver":"qcow2","file":{"driver":"gluster",\
                  "volume":"gv0","path":"sn1.qcow2",
                  "server":[{"type":\
		  "tcp","host":"10.73.199.197","port":"24007"}]},}'

Currently it will commit the content and then report an incredibly
useless error message when trying to re-open the committed image:

  qemu-img: invalid URI
  Usage: file=gluster[+transport]://[host[:port]]volume/path[?socket=...][,file.debug=N][,file.logfile=/path/filename.log]

With this fix we get:

  qemu-img: invalid URI json:{"server.0.host": "10.73.199.197",
      "driver": "gluster", "path": "luks.qcow2", "server.0.type":
      "tcp", "server.0.port": "24007", "volume": "gv0"}

Of course the root cause problem still exists, but now we know
what actually needs fixing.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180206105204.14817-1-berrange@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>