quorum: Use qapi_enum_parse() in quorum_open()

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-12-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased, qemu_opt_get() factored out, commit message tweaked]
Cc: Alberto Garcia <berto@igalia.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-9-git-send-email-armbru@redhat.com>