[qemu] Support HVM guests with more than 3.75G memory.

[qemu] Support HVM guests with more than 3.75G memory.
Changes are:
1) M2P table and e820 table are changed to skip address space from
HVM_RAM_LIMIT_BELOW_4G to 4G.
2) shared io page location, when less than HVM_RAM_LIMIT_BELOW_4G
memory, it's the last page of RAM as today, or it's the last page of
HVM_RAM_LIMIT_BELOW_4G RAM.
3) in qemu-dm address space from HVM_RAM_LIMIT_BELOW_4G to 4G are
stuffed with mfns starting from 4G, so the 1:1 mapping can still works.
This is ugly, but another limit check patch as changeset 10757 will
prevent qemu-dm to access this range.  This ugly stuffing will be
removed when the patch to remove 1:1 mapping from qemu-dm gets accepted
in the future.

Signed-off-by: Xin Li <xin.b.li@intel.com>
Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>