Purge xbus in BaseSubsystem #859

Job	Run time
riscv-arch-test	0s
riscv-tests (DefaultConfig)	11m 30s
emulator (DefaultSmallConfig)	11m 28s
riscv-tests (DefaultBufferlessConfig)	11m 32s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultC...	11m 29s
emulator (DualBankConfig)	11m 25s
riscv-tests (DefaultRV32Config)	11m 32s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.DefaultR...	11m 13s
riscv-tests (TinyConfig)	11m 30s
emulator (DualChannelConfig)	11m 29s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	11m 16s
emulator (DualChannelDualBankConfig)	11m 22s
riscv-tests (DefaultFP16Config)	11m 14s
jtag-dtm-test (freechips.rocketchip.system.WithJtagDTMSystem_freechips.rocketchip.system.WithDebu...	11m 27s
emulator (RoccExampleConfig)	11m 17s
emulator (Edge128BitConfig)	11m 21s
emulator (Edge32BitConfig)	11m 24s
emulator (QuadChannelBenchmarkConfig)	11m 34s
emulator (EightChannelConfig)	11m 29s
emulator (DualCoreConfig)	11m 28s
emulator (MemPortOnlyConfig)	11m 12s
emulator (MMIOPortOnlyConfig)	11m 29s
emulator (CloneTileConfig)	11m 30s
emulator (HypervisorConfig)	11m 29s
	4h 22m 40s

Provide feedback