Description of problem:
After upgrading engine with 4.1 and host in cluster with 4.19.4, and trying to migrate VMs with custom_compatibility_version=3.6 from other (not yet updated) hosts, getting error, i.e.:
alee-f24-ipa:
Cannot migrate VM. There is no host that satisfies current scheduling constraints. See below for details:
The host dell-r420-01 did not satisfy internal filter Compatibility-Version because it doesn't support compatibility version '3.6' which is required by the VM. Host supported compatibility versions are: 4.0,4.1..


Version-Release number of selected component (if applicable):
ovirt-engine-4.1.0.4-0.1.el7.noarch
vdsm-4.19.4-1.el7ev.x86_64

How reproducible:
Update engine and single host with 4.1, migrate vm (which have custom_compatibility_version=3.6 set) from not updated host to updated host.

Steps to Reproduce:
1.
2.
3.

Actual results:
Cannot migrate VM. There is no host that satisfies current scheduling constraints. See below for details:
The host dell-r420-01 did not satisfy internal filter Compatibility-Version because it doesn't support compatibility version '3.6' which is required by the VM. Host supported compatibility versions are: 4.0,4.1..

Expected results:
Successfull migration.

Additional info:
The workaround is to add "3.6" to 'supportedENGINEs' in /usr/lib/python2.7/site-packages/vdsm/dsaversion.py :

version_info = {
    'version_name': version_name,
    'software_version': software_version,
    'software_revision': software_revision,
    'a': ['3.6', '4.0', '4.1'],
    'clusterLevels': ['3.6', '4.0', '4.1'],
}