qga: guest_suspend: decoupling pm-utils and sys logic

Following the same logic of the previous patch, let's also
decouple the suspend logic from guest_suspend into specialized
functions, one for each strategy we support at this moment.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>