livemigrate : activate xbzrle cache

livemigrate : activate xbzrle cache

This help migrate for vm with of lot of memory access (like database)

live migration tests working:

kvm 1.2 -> kvm 1.2  (xbzrle set on both side)
kvm 1.1 -> kvm 1.2 (xbzrle on target)
kvm 1.1 -> kvm 1.1 (xbzrle not set, qmp command try to set xbzrle but fail)

failing migration

kvm 1.2 -> kvm 1.1 fail, but this is expected.

I tested with a memory benchmark running on the vm with 4GB ram

without xbzrle : migration take 10min, with many network hang
with xbzrle : migration take 1min, no hang

I display xbzrle counters for debug purpose, we can remove them later

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>