net: virtual: Fix compiler warnings related to interface name

Fix compiler warning by adjusting the number of chars copied to the
destination. Compiler does not like if the destination size of the
`strncpy`-operation is the same as the number of characters written. Even
though it is not a bug in this case. Only copying size-1 characters fixes
the warning and exhibits the same behavior.

Signed-off-by: Cla Mattia Galliard <cla-mattia.galliard@zuehlke.com>