Added device-sharing checks for loopback-mounted files. The existing

Added device-sharing checks for loopback-mounted files.  The existing
check_sharing functions have been rejigged slightly so that the file-specific
stuff can share the code therein.  This separates the actual test from the
generation of the error message, and introduces the canonicalise_mode function.

This also restores the broken teardown functionality for file: devices -- the
writing of XENBUS_PATH/node had been removed, but this is required for file
device teardown.

Signed-off-by: Ewan Mellor <ewan@xensource.com>