Add a 'Block' module for consistency with mirage-block-unix
connect: if we don't recognise the disk id, use the first available disk

For the moment mirari will pass the full path of the image in dom0.
Until we come up with a better scheme, we choose the first disk if
we don't recognise the id.
Release 0.5.0

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #13 from djs55/fix-connect

Fix connect