Add a tag slot to opcodes.OpInstanceCreate. We do not reuse _PTags, as this is
intended for OpTagsSet and thus:

  a) is not documented
  b) does not carry a default value, making it mandatory

Also pass the tags to the iallocator during instance creation.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>