virtio-net: add helper func `write_to_mmds_or_tap`
virtio-net: add helper func `write_to_mmds_or_tap`

Similar to the existing RX counter-part function
'read_from_mmds_or_tap()', this new function works for
TX to send a packet to either MMDS or the host tap.

Makes the `process_tx()` code simpler and allows
unittesting the MMDS logic.

Signed-off-by: Adrian Catangiu <43487eb7d05f4ed2cf28dd3e748821285e5f7796@amazon.com>

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