vl: Unlink absolute PID file path

After writing the PID file, we register an exit notifier to unlink it
when the process terminates.  However, if the process has changed its
working directory in the meantime (e.g. in os_setup_post() when
daemonizing), this will not work when the PID file path was relative.
Therefore, pass the absolute path (created with realpath()) to the
unlink() call in the exit notifier.

(realpath() needs a path pointing to an existing file, so we cannot use
it before qemu_write_pidfile().)

Reproducer:
$ cd /tmp
$ qemu-system-x86_64 --daemonize --pidfile qemu.pid
$ file qemu.pid
qemu.pid: ASCII text
$ kill $(cat qemu.pid)
$ file qemu.pid
qemu.pid: ASCII text

(qemu.pid should be gone after the process has terminated.)

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220609122701.17172-4-hreitz@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>