When idmap was added to LXC, we forgot to cover it in the testsuite.
The schema was missing an <element> layer, and as a result,
virt-xml-validate was failing on valid dumpxml output.

Reported by Eduard - Gabriel Munteanu on IRC.

* docs/schemas/domaincommon.rng (idmap): Include <idmap> element,
and support interleaves.
* tests/lxcxml2xmldata/lxc-idmap.xml: New file.
* tests/lxcxml2xmltest.c (mymain): Test it.
Signed-off-by: NEric Blake <eblake@redhat.com>