tests/qtest/rtl8139-test: Check whether the rtl8139 device is available

tests/qtest/rtl8139-test: Check whether the rtl8139 device is available

Though we are already using CONFIG_RTL8139_PCI in the meson.build file
for testing whether the rtl8139 device is available or not, this is not
enough: The CONFIG switch might have been selected by another target
(e.g. the mips fuloong2e machine has the rtl8139 chip soldered on the
board), so CONFIG_RTL8139_PCI ends up in config_all_devices and the
test then gets executed on x86. We need an additional run-time check
to be on the safe side to make this test also work when configure has
been run with "--without-default-devices".

Message-Id: <20230525081016.1870364-4-thuth@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Thomas Huth <thuth@redhat.com>