Add API to get memory mapping without do paging

crash does not need the virtual address and physical address mapping, and the
mapping does not include the memory that is not referenced by the page table.
crash does not use the virtual address, so we can create the mapping for all
physical memory(virtual address is always 0). This patch provides a API to do
this thing, and it will be used in the following patch.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
