new attribute accessmode to filesystem element

new attribute accessmode to filesystem element

This introduces new attribute to filesystem element
to support customizable access mode for mount type.
Valid accessmode are: passthrough, mapped and squash.

Usage:
        <filesystem type='mount' accessmode='passthrough'>
          <source dir='/export/to/guest'/>
          <target dir='mount_tag'/>
        </filesystem>

passthrough is the default model if not specified, that's
also the current behaviour.