restapi: add missing disk.export() descriptors

This patch is adding restapi rsdl metadata leftovers for the
"import/export images from/to Glance" feature

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=990962
Change-Id: I61136522a018d3633182a00f1a70e70ec00f6480
Signed-off-by: Michael pasternak <mpastern@redhat.com>