hooks: Add hook for fake VM stats

Added 4 new hook-able events:
 * before_get_vm_stats
 * after_get_vm_stats
 Called upon getVmStats() API call.

 * before_get_all_vm_stats()
 * after_get_all_vm_stats()
 Called upon getAllVmStats() API call.

Also added 'fakevmstats' hook, that is performed on
after_get_all_vm_stats event and inserts random fake statistics for the
VM.

Change-Id: I6a6d13f43495170a5cb86ae0b0efc1ae85ef05ab
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25927
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>