relaxng: allow : in /dev/disk/by-path names
    
    On IRC, Hydrar pointed a problem where 'virsh edit' failed on
    his domain created through an ISCSI pool managed by virt-manager,
    all because the XML included a block device with colons in the
    name.
    
    * docs/schemas/basictypes.rng (absFilePath): Add colon as safe.
    * tests/qemuxml2argvdata/qemuxml2argv-disk-iscsi.xml: New file.
    * tests/qemuxml2argvdata/qemuxml2argv-disk-iscsi.args: Likewise.
    * tests/qemuxml2argvtest.c (mymain): Test it.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>