memory: read host reserved/free memory via the memory control interface (ie via squeezed)

In the event that squeezed isn't running, we assume that overheads are 0
so we will attempt to start VMs and see what happens.
xapi: remove dead code

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
rrdd: need to move the datasources into the xenops interface
xapi/rrdd: convert the DS name vifX.Y into VIF uuid.Y using the xapi_xenops cache to lookup the domid

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xapi: don't ask xenopsd to manage dom0, it shouldn't be necessary

We shouldn't have any dom0 frontends. Backends of managed frontends
are themselves managed anyway, even if the backend domains are not.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
xapi: don't link against xenctrl or xenstore