crosvm: fix serial device panic on write to read-only registers
crosvm: fix serial device panic on write to read-only registers

There were a few cases of a write into the serial device that would
panic because the registers were read-only. This change simply ignores
those writes instead of raising a panic.

This is also the first bug that was found by cargo-fuzz! Wohoo!

TEST=None
BUG=None

Change-Id: I8b6805617ac3dcfddd0555c1bb394a7bc1a7cf6d
Reviewed-on: https://chromium-review.googlesource.com/518445
Commit-Ready: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm