machine_aspeed.py: update to test network for AST2700

Update test case to test network connection via SSH.

Test command:
```
cd build
pyvenv/bin/avocado run ../qemu/tests/avocado/machine_aspeed.py:AST2x00MachineSDK.test_aarch64_ast2700_evb_sdk_v09_02
```

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
1 file changed