qga: refactoring qmp_guest_suspend_* functions

To be able to add new suspend mechanisms we need to detach
the existing QMP functions from the current implementation
specifics.

At this moment we have functions such as qmp_guest_suspend_ram
calling bios_suspend_mode and guest_suspend passing the
pmutils command and arguments as parameters. This patch
removes this logic from the QMP functions, moving them to
the respective functions that will have to deal with which
binary to use.

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