Migration: warn the user about hv version mismatch

* hv_kvm.py, hv_xen.py
  - return the hypervisor version (if available) from GetNodeInfo

* cmdlib.py
  - if hypervisor version is available during the migration, and the
    versions differ, warn the user

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>