Redirect crosvm stdin to /dev/null

If this is not done crosvm inherits stdin from the launcher, which, if
run in foreground could lead to accidentally inserting unwanted
commands in the console. Additionally, crosvm attempts to control the
terminal it's running in, which causes it to hang when run in
background.

Bug: 128457282
Test: run locally
Change-Id: I2c7d4b24f7c614af5bea32964c7f609b6c140092