tools/examples/block better use of losetup -r

tools/examples/block better use of losetup -r

Use losetup -r when the vbd is specified to be readonly, but only if
-r is supported (rather than always creating a writeable losetup
mapping if possible).

This was inspired by (but not derived from) a diff from the Fedora 8
patchset which uses -r iff the vbd is specified as readonly.  We need
to be cleverer upstream because not all systems have losetup -r.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>