[RHV] Migration fails during precopy/cutover while a snapshot operation is executed on the source VM

This issue is not specific to the case that another snapshot operation is triggered, it could also happen when, e.g., the vm is migrated. We need to make the migration flow more robust.
Description of problem:
Warm migration fails on the following error in the case that another snapshot operation (create/delete) is executed on the RHV source VM:

Fault reason is "Operation Failed". Fault detail is "[Cannot create Snapshot. The VM is performing an operation on a Snapshot. Please wait for the operation to finish, and try again.]". HTTP response code is "409". HTTP response message is "409 Conflict".

Version-Release number of selected component (if applicable):
MTV 2.3.0-32 / iib:178080
CNV 4.10.0-686 / iib:178572

How reproducible:
100%

Steps to Reproduce:
1. Start a warm migration plan using RHV VM
2. Create a snapshot on the source VM near the next precopy or the cutover time

Actual results:
Migration fails

Expected results:
MTV should wait for the snapshot operation to finish in order to run the next precopy/cutover

Original bug: https://bugzilla.redhat.com/show_bug.cgi?id=2057459