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>

tests/avocado/boot_linux_console.py

@@ -381,6 +381,8 @@ class BootLinuxConsole(LinuxKernelTest):
         :avocado: tags=u-boot
         :avocado: tags=accel:tcg
         """
+        self.require_netdev('user')
+
         uboot_url = ('https://raw.githubusercontent.com/'
                      'Subbaraya-Sundeep/qemu-test-binaries/'
                      'fe371d32e50ca682391e1e70ab98c2942aeffb01/u-boot')
@@ -779,6 +781,8 @@ class BootLinuxConsole(LinuxKernelTest):
         :avocado: tags=machine:orangepi-pc
         :avocado: tags=device:sd
         """
+        self.require_netdev('user')
+
         deb_url = ('https://apt.armbian.com/pool/main/l/'
                    'linux-5.10.16-sunxi/linux-image-current-sunxi_21.02.2_armhf.deb')
         deb_hash = '9fa84beda245cabf0b4fa84cf6eaa7738ead1da0'