qemu-ga: ga_open_pidfile(): add new line to pidfile

FHS requires that pid files should end with a new line. Also change to
snprintf() while there.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
