Move the dom0 ballooning-out code from the xm create handler and into the

Move the dom0 ballooning-out code from the xm create handler and into the
server, where it can be used on reboot and migrate as well as create.
Ballooning bugs may still exist (it is unclear) but this is certainly a good
step in the right direction.

Ignore the memory and maxmem entries in the store for dom0 when restarting.
This means that the value in use at the time becomes the configured target.
This should fix the bug whereby dom0_mem settings on the command line are
being overridden by (older) entries in the store.

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