qemu: Fix updating device with boot order
    
    Commit v3.7.0-14-gc57f3fd2f8 prevented adding a <boot order='x'/>
    element to an inactive domain with global <boot dev='...'/> element.
    However, as a result of that change updating any device with boot order
    would fail with 'boot order X is already used by another device', where
    "another device" is in fact the device which is being updated.
    
    To fix this we have to ignore the device which we're about to update
    when checking for boot order conflicts.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1546971
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>