savevm-async: set SAVE_STATE_DONE when closing state file was successful

savevm-async: set SAVE_STATE_DONE when closing state file was successful

Without this change, it's necessary to send a second savevm-end QMP
command after aborting a snaphsot, before a new savevm-start QMP
command can succeed.

In process_savevm_finalize(), no longer set an error in the abort
scenario. If there already is another error, there's no need to
override it. If canceling was done intentionally, qmp_savevm_end()
is responsible for setting the state now.

Reported-by: Mira Limbeck <m.limbeck@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>