env_process: relocate migrate vm registration

In the preprocess routine from env_process a piece of code is in charge
of registering the "migrate_vms" into the "vm" configuration parameter.
Some of the environment preprocessing steps might need from that
configuration to work properly. However, due to the ongoing refactoring
of the env_process:{pre,post}process functions, the migrate_vms might be
registered after the environment setup step. This will probably make
some of the setup steps misbehave.

For that reason, let's move the migrate_vm registration step to the
beginning of the preprocess function.

Signed-off-by: Beñat Gartzia Arruabarrena <bgartzia@redhat.com>

virttest/env_process.py

@@ -1003,6 +1003,12 @@ def preprocess(test, params, env):
     if callable(preprocess_vm_off_hook):
         preprocess_vm_off_hook(test, params, env)  # pylint: disable=E1102
 
+    # Add migrate_vms to vms
+    migrate_vms = params.objects("migrate_vms")
+    if migrate_vms:
+        vms = list(set(params.objects("vms") + migrate_vms))
+        params["vms"] = " ".join(vms)
+
     # Check if code coverage for qemu is enabled and
     # if coverage reset is enabled too, reset coverage report
     gcov_qemu = params.get("gcov_qemu", "no") == "yes"
@@ -1102,12 +1108,6 @@ def preprocess(test, params, env):
 
     base_dir = data_dir.get_data_dir()
 
-    # Add migrate_vms to vms
-    migrate_vms = params.objects("migrate_vms")
-    if migrate_vms:
-        vms = list(set(params.objects("vms") + migrate_vms))
-        params["vms"] = " ".join(vms)
-
     # Permit iptables to permit 49152-49216 ports to libvirt for
     # migration and if arch is ppc with power8 then switch off smt
     # will be taken care in remote machine for migration to succeed