tests/qtest/npcm7xx_emc-test: Connect all NICs to a backend

tests/qtest/npcm7xx_emc-test: Connect all NICs to a backend

Currently QEMU will warn if there is a NIC on the board that
is not connected to a backend. By default the '-nic user' will
get used for all NICs, but if you manually connect a specific
NIC to a specific backend, then the other NICs on the board
have no backend and will be warned about:

qemu-system-arm: warning: nic npcm7xx-emc.1 has no peer
qemu-system-arm: warning: nic npcm-gmac.0 has no peer
qemu-system-arm: warning: nic npcm-gmac.1 has no peer

So suppress those warnings by manually connecting every NIC
on the board to some backend.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20240206171231.396392-3-peter.maydell@linaro.org