domain_conf: fix internal flag verification
    
    While working on virDomainBlockCopy, I noticed we had a verify()
    concerning internal XML flags that was incomplete after several
    recent flag additions; move that up higher in the code to make it
    harder to forget to modify on the next flag addition.  Adjust
    some formatting while at it.
    
    * src/conf/domain_conf.c (verify): Move closer to internal flag
    definitions.  Cover missing flags ALLOW_ROM and ALLOW_BOOT.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>