hmp: Use qapi_enum_parse() in hmp_migrate_set_capability()

hmp: Use qapi_enum_parse() in hmp_migrate_set_capability()

The error message on invalid capability name changes from

    Invalid parameter "NAME"

to

    invalid parameter value: NAME

No worse than before.

Signed-off-by: Marc-André Lureau <[email protected]>
Message-Id: <20170822132255[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
[Rebased, commit message rewritten]
Cc: "Dr. David Alan Gilbert" <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Message-Id: <1503564371[email protected]>