schema: Re-structure schema for <filesystem> to avoid broken validation

schema: Re-structure schema for <filesystem> to avoid broken validation

The validation of a '<filesystem type='mount'>' device fails if the
elements inside are not ordered in the order in the schema despite using
<interleave>. This is a bug in libxml2's validator as removing the
'<optional>' property from the definition of the 'type' attribute with
'mount' variable fixes the problem.

I've reported it as another instance of a seemingly related issue:

  https://gitlab.gnome.org/GNOME/libxml2/-/issues/131

Meanwhile libvirt can re-arrange the schema by extracting the common
bits into a new definition and referencing them from each of the choice
groups explicitly.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/392
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>