Remove VIR_STRNDUP usage that subtracts from a non-NULL pointer

Use g_strndup in all the cases where we check upfront whether a pointer
is non-NULL and then use it to calculate the copied length.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>