w32: Fix qemu_ftruncate64

SetFilePointer returns INVALID_SET_FILE_POINTER when it fails.
In addition, GetLastError must be checked.

The first call of SetFilePointer did not use INVALID_SET_FILE_POINTER,
the second call used wrong error handling.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
