rumprun: Make diskspec backward compatible with Xen 4.1

Xen 4.1 is used in Debian stable; the diskspec used by xl is not clearly
documented and is just a passthrough to the xm disk stanza. We need mode
'w' rather than 'rw' which is also understood by (at least) Xen 4.4.

Signed-off-by: Martin Lucina <martin@lucina.net>
rumprun: Use absolute path for disk image

Avoids potential Xen stack confusion (documentation is unclear on
relative paths), plus shows full path in xenstore-ls.

Signed-off-by: Martin Lucina <martin@lucina.net>
rumprun: Use blkid rather than file to detect fstype

The output of file differs between systems (and Linux distribution
versions!). Use blkid for now to detect block device filesystem type,
will need to be revisited or marked as a dependency on e2fsprogs for
*BSD.

Signed-off-by: Martin Lucina <martin@lucina.net>