domain_conf: fix internal flag verification

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>