spapr: Implement ibm,suspend-me

spapr: Implement ibm,suspend-me

This has been useful to modify and test the Linux pseries suspend
code but it requires modification to the guest to call it (due to
being gated by other unimplemented features). It is not otherwise
used by Linux yet, but work is slowly progressing there.

This allows a (lightly modified) guest kernel to suspend with
`echo mem > /sys/power/state` and be resumed with system_wakeup
monitor command.

Signed-off-by: Nicholas Piggin <[email protected]>
Message-Id: <20190722061752[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Signed-off-by: David Gibson <[email protected]>