Implement SR.scan

Every regular file in the SR's directory is considered to be a valid
VDI. If the file is accompanied by a '.json' metadata file then this
will be consulted for metadata; otherwise fresh data will be created.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Make sure we don't consider the .json metadata files as virtual disks

Signed-off-by: David Scott <dave.scott@eu.citrix.com>