This permits configuration with driver-specific options in addition to
pseudo-filename parsed as URI.  For instance,

    --drive driver=sheepdog,host=fido,vdi=dolly

instead of

    --drive driver=sheepdog,file=sheepdog://fido/dolly

It's also a first step towards supporting blockdev-add.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>