crosvm: fix removing comments when processing -file options
crosvm: fix removing comments when processing -file options

As it turns out trim_end_matching() does not do what I expected (namely
trim the string when it encounters matching substring) but rather
expects to remove exactly matching suffix, so it actually fails to
remove comments. Let's switch to using splitn() that does what I want.

BUG=b:144454617
TEST=Run Plugin VM

Change-Id: Ibe8cd14d9d3c14ff99dd9abfa8f286e452d96913
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1975095
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Commit-Queue: Dmitry Torokhov <10a8c465cefc9bdd6c925e26964d23c90f1141cc@chromium.org>
Reviewed-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>

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