qemu_passt: Precreate passt logfile

qemu_passt: Precreate passt logfile

There are a few situations where passt itself is unable to create
a file because it runs under QEMU user (e.g. just like our
example from formatdomain.rst suggests: /var/log/passt.log). If
libvirtd runs with sufficient permissions (e.g. as root) it can
create the file and set seclabels on it so that passt can then
open it.

Ideally, we would just pass pre-opened FD, but this wasn't viewed
as secure enough [1]. So lets just create the file and set
seclabels.

For the case when both libvirtd and passt have the same
permissions, well then we fail before even needing to fork() and
exec().

1: https://archives.passt.top/passt-dev/20230606225836.63aecebe@elisabeth/
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2209191
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>