vz: add memory statistics
    
    Implemented counters:
     VIR_DOMAIN_MEMORY_STAT_SWAP_IN
     VIR_DOMAIN_MEMORY_STAT_SWAP_OUT
     VIR_DOMAIN_MEMORY_STAT_MINOR_FAULT
     VIR_DOMAIN_MEMORY_STAT_MAJOR_FAULT
     VIR_DOMAIN_MEMORY_STAT_AVAILABLE
     VIR_DOMAIN_MEMORY_STAT_ACTUAL_BALLOON
     VIR_DOMAIN_MEMORY_STAT_UNUSED
    
    Comments.
    
    1. Use vzDomObjFromDomainRef/virDomainObjEndAPI pair to get domain
    object as we use prlsdkGetStatsParam. See previous statistics
    comments.
    
    2. Balloon statistics is not applicable to containers. Fault
    statistics for containers not provided in PCS6 yet.
    
    Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>