stdlib: Move Root obj creation from Simulator to Board

It makes much more sense for the Root Object to be create within the
board and passed where required. Creating it in the Simulator class is
not required.

For this to work the signuature of the `_pre_instantiate` function in
`AbstractBoard` has been updated to return the Root object.

tests/gem5/kvm_fork_tests/configs/boot_kvm_fork_run.py

@@ -207,15 +207,15 @@ print("Running with ISA: " + processor.get_isa().name)
 print("Running with protocol: " + get_runtime_coherence_protocol().name)
 print()
 
-root = Root(full_system=True, system=motherboard)
+# Disable the gdb ports. Required for forking.
+m5.disableAllListeners()
+root = motherboard._pre_instantiate()
 
 # TODO: This of annoying. Is there a way to fix this to happen
 # automatically when running KVM?
 root.sim_quantum = int(1e9)
 
-# Disable the gdb ports. Required for forking.
-m5.disableAllListeners()
-motherboard._pre_instantiate()
+
 m5.instantiate()
 
 # Simulate the inital boot with the starting KVM cpu

tests/gem5/replacement_policies/configs/run_replacement_policy.py

@@ -83,9 +83,8 @@ motherboard = TestBoard(
     memory=memory,
     cache_hierarchy=cache_hierarchy,
 )
-root = Root(full_system=False, system=motherboard)
 
-motherboard._pre_instantiate()
+root = motherboard._pre_instantiate()
 m5.instantiate()
 
 generator.start_traffic()

tests/gem5/replacement_policies/run_replacement_policy.py

@@ -83,9 +83,8 @@ motherboard = TestBoard(
     memory=memory,
     cache_hierarchy=cache_hierarchy,
 )
-root = Root(full_system=False, system=motherboard)
 
-motherboard._pre_instantiate()
+root = motherboard._pre_instantiate()
 m5.instantiate()
 
 generator.start_traffic()

tests/gem5/traffic_gen/configs/simple_traffic_run.py

@@ -202,9 +202,7 @@ motherboard = TestBoard(
     cache_hierarchy=cache_hierarchy,
 )
 
-root = Root(full_system=False, system=motherboard)
-
-motherboard._pre_instantiate()
+root = motherboard._pre_instantiate()
 m5.instantiate()
 
 generator.start_traffic()