Description of problem:
Upgrade of schema fails when upgrading to spacewalk nightly

Steps to Reproduce:
1. have installed Spacewalk 2.4
2. enable spacewalk nightly repo
3. update spacewalk-schema package
4. execute command spacewalk-schema-upgrade

Actual results:
# spacewalk-schema-upgrade 
Schema upgrade: [spacewalk-schema-2.4.24-1.el7] -> [spacewalk-schema-2.5.19-1.el7]
Searching for upgrade path: [spacewalk-schema-2.4.24-1] -> [spacewalk-schema-2.5.19-1]
Searching for upgrade path: [spacewalk-schema-2.4.24] -> [spacewalk-schema-2.5.19]
Searching for upgrade path: [spacewalk-schema-2.4] -> [spacewalk-schema-2.5]
The path: [spacewalk-schema-2.4] -> [spacewalk-schema-2.5]
Planning to run spacewalk-sql with [/var/log/spacewalk/schema-upgrade/20160322-061140-script.sql]

Plase make sure you have a valid backup of your database before continuing.

Hit Enter to continue or Ctrl+C to interrupt: 
Executing spacewalk-sql, the log is in [/var/log/spacewalk/schema-upgrade/20160322-061140-to-spacewalk-schema-2.5.log].
Upgrade failed, please see log [/var/log/spacewalk/schema-upgrade/20160322-061140-to-spacewalk-schema-2.5.log].

Additional info:
Part of log file containing error:

                                          ?column?
--------------------------------------------------------------------------------------------
 spacewalk-schema-2.4-to-spacewalk-schema-2.5/167-remove_virt_sub_level_refs.sql.postgresql
(1 řádka)

DELETE 1
DELETE 1
psql:/var/log/spacewalk/schema-upgrade/20160322-061140-script.sql:9331: ERROR:  update or delete on table "rhnvirtsublevel" violates foreign key constraint "rhn_cfvsl_vslid_fk" on table "rhnchannelfamilyvirtsublevel"
DETAIL:  Key (id)=(1) is still referenced from table "rhnchannelfamilyvirtsublevel".

RHEL 7 channel synced.