Fix compile warnings

Fix compile warnings

Fix two warnings:

e:\xeniface\src\xenagent\service.cpp(684): warning C26439: This kind offunction may not throw.
Declare it 'noexcept' (f.6). [E:\xeniface\vs2017\xenagent\xenagent.vcxproj]

e:\xeniface\src\xenagent\xenifacedevice.cpp(57): warning C26451: Arithmetic overflow: Using operator
'+' on a 4 byte value and then casting the result to a 8 byte value. Cast the value to the wider type before
calling operator '+' to avoid overflow (io.2). [E:\xeniface\vs2017\xenagent\xenagent.vcxproj]

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>