Append newline to all QGA commands

Append newline to all QGA commands

Since the MacOS Mojave Apple ships AppleQEMUGuestAgent by default.
However, it does not fully adhere to QGA specs as they do expect each
command to be newline delimited.

This makes each command to be newline delimited which is harmless for
all other systems (Windows, Linux), but enable guest agent by default
without any changes on OSX.

Signed-off-by: Kamil Trzcinski <ayufan@ayufan.eu>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>