[Virtio-fs] [virtiofsd] MR opened: Server: Store enabled FSOptions

[virtiofs-bot at sinrega.org]

Store enabled FSOptions as Server field and get the correct structure based on whether SETXATTR_EXT is set in the negotiated FSOptions flags.

Also, pass the 'SetxattrIn::setxattr_flags' as extra flags to 'FileSystem::setxattr', currently this argument is ignored, but will be required to implement posix_acls and selinux support.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/92