[XEND] separate concept of initial memory size and overhead memory size.

[XEND] separate concept of initial memory size and overhead memory size.

When a domain (whether para- or fully-virtualized) reports how much
overhead memory it requires (via getDomainMemory in image.py), all such
memory was immediately allocated to the domain itself.  This is
certainly incorrect for HVM domains, since additional
increase_reservation calls are made later in qemu.  Since all ballooned
memory is already taken, qemu will fail.  The fix is to treat the
requested memory size and the overhead size as separate values.  The
requested memory size is immediately allocated to the new domain; the
overhead is left unallocated for whatever else might need it later.

Signed-off-by: Charles Coffing <ccoffing@novell.com>