Fixed null pointer after deleting snapshot, GC and cross cluster vm migration on XCP-NG

[Spaceman1984]

Description

Fixes #4090

When trying to migrate a VM across 2 clusters, if a snapshot has been deleted and garbage collection has run to update the removed field, it is not possible to migrate the instance due to a null pointer.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)

Screenshots (if appropriate):

How Has This Been Tested?

This has been tested by migrating an instance to another Xenserver cluster using XCP-NG as the hypervisor in both clusters.

Process to follow:

• Create a zone with 2 clusters.
• Create an instance in 1 cluster.
• Take a root volume snapshot of the instance.
• Delete the snapshot.
• Wait for the removed field to be populated in the snapshots table.
• Migrate the instance.

[rohityadavcloud]

@Spaceman1984 could you run package and test yourself for your PRs in future?
@blueorangutan package

[blueorangutan]

@rhtyd a Jenkins job has been kicked to build packages. I'll keep you posted as I make progress.

[blueorangutan]

Packaging result: ✔centos7 ✔debian. JID-1299

[rohityadavcloud]

@blueorangutan test

[blueorangutan]

@rhtyd a Trillian-Jenkins test job (centos7 mgmt + kvm-centos7) has been kicked to run smoke tests

[rohityadavcloud]

@blueorangutan test

[blueorangutan]

@rhtyd a Trillian-Jenkins test job (centos7 mgmt + kvm-centos7) has been kicked to run smoke tests

[blueorangutan]

Trillian test result (tid-1701)
Environment: kvm-centos7 (x2), Advanced Networking with Mgmt server 7
Total time taken: 32371 seconds
Marvin logs: https://github.com/blueorangutan/acs-prs/releases/download/trillian/pr4130-t1701-kvm-centos7.zip
Intermittent failure detected: /marvin/tests/smoke/test_certauthority_root.py
Intermittent failure detected: /marvin/tests/smoke/test_internal_lb.py
Intermittent failure detected: /marvin/tests/smoke/test_privategw_acl.py
Intermittent failure detected: /marvin/tests/smoke/test_routers_network_ops.py
Smoke tests completed. 76 look OK, 1 have error(s)
Only failed tests results shown below:

Test	Result	Time (s)	Test File
test_02_vpc_privategw_static_routes	Failure	184.32	test_privategw_acl.py
test_03_vpc_privategw_restart_vpc_cleanup	Failure	182.01	test_privategw_acl.py
test_04_rvpc_privategw_static_routes	Failure	250.19	test_privategw_acl.py

[shwstppr]

LGTM, makes sense for preventing NPE