XBZRLE: Fix qemu crash when resize the xbzrle cache

Resizing the xbzrle cache during migration causes qemu-crash,
because the main-thread and migration-thread modify the xbzrle
cache size concurrently without lock-protection.

Signed-off-by: ChenLiang <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
