xend: Memory pool for pv guest on systems with >128G memory

xend: Memory pool for pv guest on systems with >128G memory

The main idea of this patch is:

1) The admin sets aside some memory below 128G for 32-bit paravirtual
domain creation (via dom0_mem=-<value> in kernel comand line).

2) The admin also explicitly states to the tools (i..e xend) how much
memory is supposed to be left untouched by 64-bit domains

3) If a 32-bit pv DomU gets created, no ballooning ought to be
necessary (since if it is, no guarantee can be made about the address
range of the memory ballooned out), and memory gets allocated from the
reserved range.

4) Upon 64-bit (or 32-bit HVM or HVM) DomU creation, the tools
determine the amount of memory to be ballooned out of Dom0 by adding
the amount needed for the new guest and the amount still in the
reserved pool (and then of course subtracting the total amount of
memory the hypervisor has available for guest use).

Signed-off-by: james song (wei) <jsong@novell.com>