When using memory spaces, a backend may sometimes qualify an address
to specify the memory space to which the address belongs. However, to
minimize impacts, the memory space qualifier may not always be present.
In such cases, the address is implied to belong to the "default" memory
space.
DSF-GDB currently has no means to know what is the "default" memory
space to apply in such cases.
This change introduces a new API to request the default memory space
from the memory service.
Change-Id: Icea87543b8529756aee92c6d2af95fb936f5fb9c
2655b4a0f3