qga: add channel path to error messages

It's useful to know which device was used if/when it fails.

channel-win32.c had this since 2015, with
c69403fc ("qemu-ga: debug printouts to
help troubleshoot installation"), this brings channel-posix.c up to
speed.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>