selftests: sev_migrate_tests: free all VMs

Ensure that the ASID are freed promptly, which becomes more important
when more tests are added to this file.

Cc: Peter Gonda <pgonda@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>