Fs_block serves an image from a file system server over a Block session.
Only one session is served at a time. Configuration is straightforward;
images are not writeable by default.

! <config>
!   <default-policy file="/image.raw" block_size="512" writeable="yes""/>
! </config>

! <config>
!   <default-policy file="/disk" writeable="yes" device_size="64G"/>
! </config>

! <config>
!   <default-policy file="/image.iso" block_size="2048"/>
! </config>
