migration: Convert 'status' of MigrationInfo to use an enum type

migration: Convert 'status' of MigrationInfo to use an enum type

The original 'status' is an open-coded 'str' type, convert it to use an
enum type.
This conversion is backwards compatible, better documented and
more convenient for future extensibility.

In addition, Fix a typo for qapi-schema.json (just remove the typo) :
s/'completed'. 'comppleted' (since 1.2)/'completed' (since 1.2)

Signed-off-by: zhanghailiang <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>