sheepdog: Report errors in pseudo-filename more usefully

Errors in the pseudo-filename are all reported with the same laconic
"Can't parse filename" message.

Add real error reporting, such as:

    $ qemu-system-x86_64 --drive driver=sheepdog,filename=sheepdog:///
    qemu-system-x86_64: --drive driver=sheepdog,filename=sheepdog:///: missing file path in URI
    $ qemu-system-x86_64 --drive driver=sheepdog,filename=sheepgod:///vdi
    qemu-system-x86_64: --drive driver=sheepdog,filename=sheepgod:///vdi: URI scheme must be 'sheepdog', 'sheepdog+tcp', or 'sheepdog+unix'
    $ qemu-system-x86_64 --drive driver=sheepdog,filename=sheepdog+unix:///vdi?socke=sheepdog.sock
    qemu-system-x86_64: --drive driver=sheepdog,filename=sheepdog+unix:///vdi?socke=sheepdog.sock: unexpected query parameters

The code to translate legacy syntax to URI fails to escape URI
meta-characters.  The new error messages are misleading then.  Replace
them by the old "Can't parse filename" message.  "Internal error"
would be more honest.  Anyway, no worse than before.  Also add a FIXME
comment.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
