virtio_fs: allow idmapped mounts

Allow idmapped mounts for virtiofs.
It's absolutely safe as for virtiofs we have the same
feature negotiation mechanism as for classical fuse
filesystems. This does not affect any existing
setups anyhow.

virtiofsd support:
https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/245



Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>