Allow the Linux guest agent to attempt each of the suspend methods
(systemctl, pm-* and writing to /sys) in turn.

Prior to this guests without systemd failed to suspend due to
`guest_suspend` returning early regardless of the return value of
`systemd_supports_mode`.

Signed-off-by: Mark Somerville <mark@qpok.net>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>