vmstate snapshot : implement cancelled state V2

currently, if we cancel a active savevm job with savevm-end,
we don't close the vmstate file.

This implement a new state CANCELLED, which is setup if savevm-end is called
when savevm is ACTIVE

then, in the coroutine, if state CANCELLED is detected,
we call save_snapshot_completed() to properly close the file
then qmp_savevm_end to finish the job

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>