Implement get_host_uptime for powervm driver

This patch implements the get_host_uptime method for the powervm driver
by calling the sysstat command on the backing hypervisor and returning
the parsed results.

Fixes bug 1191785

Change-Id: I67aaf6a5f5eb6b3a411ca9a0284d9a3016dd2947
Merge "Implement get_host_uptime for powervm driver"