works like qemu_strtoul(). I'd prefer longhand for qemu_strtod(). It
costs us a few lines, but it results in a clearer contract.
/**
* Convert string @nptr to a double.
*
* This is a wrapper around strtod() that is harder to misuse.
* Semantics of @nptr and @endptr match strtod() with differences
* noted below.
*
* @nptr may be null, and no conversion is performed then.
*
* If no conversion is performed, store @nptr in address@hidden and return
* -EINVAL.
*
* If @endptr is null, and the string isn't fully converted, return
* -EINVAL. This is the case when the pointer that would be stored in
* a non-null @endptr points to a character other than '\0'.
*
* If the conversion overflows @result, store +/-HUGE_VAL, depending on
* the sign, in @result and return -ERANGE.