Relax size checks for integer types

I recenetly came across a Windows XP image, where one REG_QWORD value
(HKLM\Software\Microsoft\Windows\CurrentVersion\Group Policy\State\Machine\Scripts\Shutdown\0\0\ExecTime)
would be displayed by hivexsh but hivex_value_qword() would return -1.

It turned out that the data length of this value was 16 bytes instead
of 8.

There is no problem in simply interpreting the first 4 (DWORD) or
8 (QWORD) bytes -- if there are enough bytes to be interpreted.