Suggest using filters if there are more than 10 hosts.

Suggest using filters if there are more than 10 hypervisors
 to collect from.

Added:
* MAX_WARN_HOSTS_COUNT - to define max hypervisors count.

Change-Id: I05fad1b52c1672b45d0199091cb60b9642581621
Bug-Url: https://bugzilla.redhat.com/1015022
Signed-off-by: Kiril Nesenko <knesenko@redhat.com>