selftests: check hot-pluggagble memory for memory-hotplug test

selftests: check hot-pluggagble memory for memory-hotplug test

BugLink: http://bugs.launchpad.net/bugs/1710868
Check for hot-pluggable memory availability in prerequisite() of the
memory-hotplug test.

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
(cherry picked from commit a34b28c92ec8c92938de03c18c5fab32efd2e29a)
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>