The VM metadata export handler (/export_metadata) will output the metadata

The VM metadata export handler (/export_metadata) will output the metadata
of a VM, all its snapshots, and all their attached VDIs.  However, it doesn't
walk the tree of VDIs defined by the VDI.sm_config["vhd_parent"] field.  For
a full-fidelity representation, having the parent VDIs is very useful.

This patch adds an "include_vhd_parents=(true|false)" option to the the
/export_metadata handler.  With this option on, the entire tree of VDIs is
exported.

This patch does not change the /export URL handler, does not change any
behaviour when include_vhd_parents is false or missing, and copes gracefully
with a missing or malformed vhd_parent entry.

Dave: modified to be robust to vhd parent cycles

Signed-off-by: Ewan Mellor <ewan.mellor@eu.citrix.com>
Signed-off-by: Dave Scott <dave.scott@eu.citrix.com>