docs: document/mark-up SCHEDOP_*

docs: document/mark-up SCHEDOP_*

The biggest subtlety here is there additional argument when op ==
SCHEDOP_shutdown and reason == SHUTDOWN_suspend and its interpretation by
xc_domain_{save,restore}. Add some clarifying comments to libxc as well.

This patch moves some structs around but there is no functional change
other than improved documentation.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>