schema: fix idmap validation
    
    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: Eric Blake <eblake@redhat.com>