qga: add an optional qemu-ga.conf system configuration

qga: add an optional qemu-ga.conf system configuration

Learn to configure the agent with a system configuration.

This may simplify command-line handling, especially when the blacklist
is long.

Among the other benefits, this may standardize the configuration of an
init service (instead of distro-specific init keys/files)

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Michael Roth <[email protected]>
Reviewed-by: Denis V. Lunev <[email protected]>
* removed unecessary keyfile != NULL prior to free
Signed-off-by: Michael Roth <[email protected]>