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 <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-9-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased, commit message rewritten]
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-6-git-send-email-armbru@redhat.com>