dev: Use the correct return type for disk offsets

Replace the use of off_t in the various DiskImage related classes with
std::streampos. off_t is a signed 32 bit integer on most 32-bit
systems, whereas std::streampos is normally a 64 bit integer on most
modern systems. Furthermore, std::streampos is the type used by
tellg() and seekg() in the standard library, so it should have been
used in the first place. This patch makes it possible to use disk
images larger than 2 GiB on 32 bit systems with a modern C++ standard
library.

src/dev/disk_image.cc

@@ -86,7 +86,7 @@ RawDiskImage::close()
     stream.close();
 }
 
-off_t
+std::streampos
 RawDiskImage::size() const
 {
     if (disk_size == 0) {
@@ -99,8 +99,8 @@ RawDiskImage::size() const
     return disk_size / SectorSize;
 }
 
-off_t
-RawDiskImage::read(uint8_t *data, off_t offset) const
+std::streampos
+RawDiskImage::read(uint8_t *data, std::streampos offset) const
 {
     if (!initialized)
         panic("RawDiskImage not initialized");
@@ -120,8 +120,8 @@ RawDiskImage::read(uint8_t *data, off_t offset) const
     return stream.tellg() - pos;
 }
 
-off_t
-RawDiskImage::write(const uint8_t *data, off_t offset)
+std::streampos
+RawDiskImage::write(const uint8_t *data, std::streampos offset)
 {
     if (!initialized)
         panic("RawDiskImage not initialized");
@@ -357,12 +357,12 @@ CowDiskImage::writeback()
     }
 }
 
-off_t
+std::streampos
 CowDiskImage::size() const
 { return child->size(); }
 
-off_t
-CowDiskImage::read(uint8_t *data, off_t offset) const
+std::streampos
+CowDiskImage::read(uint8_t *data, std::streampos offset) const
 {
     if (!initialized)
         panic("CowDiskImage not initialized");
@@ -381,8 +381,8 @@ CowDiskImage::read(uint8_t *data, off_t offset) const
     }
 }
 
-off_t
-CowDiskImage::write(const uint8_t *data, off_t offset)
+std::streampos
+CowDiskImage::write(const uint8_t *data, std::streampos offset)
 {
     if (!initialized)
         panic("RawDiskImage not initialized");