samples/net/virtual: Increase sprintk buffers to hold parameter types

This sample app uses a stack buffer to hold the interface name which is
generated at runtime using sprintk. The buffer is only sized for the
expected range of values, not the full possible range of the datatype
(int).

To mitigate this, increase the size of the buffer to hold the full range of
an int value.

Signed-off-by: Keith Packard <keithp@keithp.com>