tests/avocado: Add missing require_netdev('user') checks

tests/avocado: Add missing require_netdev('user') checks

Some avocado tests fail if QEMU was built without libslirp. Add
require_netdev('user') checks where necessary:

These tests try to ping 10.0.2.2 and expect it to succeed:
  boot_linux_console.py:BootLinuxConsole.test_arm_emcraft_sf2
  boot_linux_console.py:BootLinuxConsole.test_arm_orangepi_sd
  ppc_bamboo.py:BambooMachine.test_ppc_bamboo

These tests run a commandline that includes '-net user':
  machine_aspeed.py:AST2x00Machine.test_arm_ast2500_evb_builroot
  (and others that use the do_test_arm_aspeed_buidroot_start()
  or do_test_arm_aspeed_sdk_start() helper functions)

These changes seem to be sufficient for 'make check-avocado'
to not fail on a --disable-slirp build.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20221001195224.2453581-1-peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>