Bug Fix: Wrong host available memory value

Host backend was using psutil.phymem* functions which are deprecated.
Also, 'available' memory has the same value of 'free' memory.
The function psutil.virtmem_usage() gives the right values, including
the amount of memory available (free + cached + buffers).

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>