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feat(console): redirect the guest console to a PTY

`--console pty` connects the guest console to a pseudo-terminal, so a
user can attach to it from another terminal, e.g. `screen /dev/pts/7`.
The slave path is logged, and `link=<path>` creates a symbolic link to
it at a stable path.

The VMM keeps the slave open itself. Otherwise, on Linux, the master
hangs up when the last client closes the slave, and reads fail with
EIO.

Writes from the guest never block. When the PTY buffer is full, the
oldest output that nobody has read is dropped, so a client attaching
late sees the newest output, e.g. the shell prompt. To do so, the VMM
discards the unread output with tcflush() and writes back the newest
512 bytes, which it keeps in a small buffer.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

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