Expose more guest stats for vROPS to report the exact physical memory

Expose more guest stats for vROPS to report the exact physical memory
stats as the guest OS UI does.

Windows:
guest.mem.modifiedPages (Win32_PerfRawData_PerfOS_Memory#ModifiedPageListBytes)
guest.mem.standby.core (Win32_PerfRawData_PerfOS_Memory#StandbyCacheCoreBytes)
guest.mem.standby.normal (Win32_PerfRawData_PerfOS_Memory#StandbyCacheNormalPriorityBytes)
guest.mem.standby.reserve (Win32_PerfRawData_PerfOS_Memory#StandbyCacheReserveBytes)

Linux:
guest.mem.total (/proc/meminfo#MemTotal)
guest.mem.buffers (/proc/meminfo#Buffers)
guest.mem.cached (/proc/meminfo#Cached)
guest.mem.slabReclaim (/proc/meminfo#SReclaimable)
guest.mem.available (/proc/meminfo#MemAvailable since Linux 3.14, emulated by VMTools for kernels 2.6.27+)