Add qemu_ram_alloc_from_ptr function

Provide a function to add an allocated region of memory to the qemu RAM.

This patch is copied from Marcelo's qemu_ram_map() in qemu-kvm and given the
clearer name qemu_ram_alloc_from_ptr().

Signed-off-by: Cam Macdonell <cam@cs.ualberta.ca>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>