Device: Consider VSA when calculating logical disk size

When the underlying disk driver has VSA access the total size must be
calculated with respect to the VSA offset.

include/icsneo/disk/diskdriver.h

@@ -33,6 +33,7 @@ public:
 	virtual std::pair<uint32_t, uint32_t> getBlockSizeBounds() const = 0;
 
 	void setVSAOffset(uint64_t offset) { vsaOffset = offset; }
+	uint64_t getVSAOffset() const { return vsaOffset; }
 
 protected:
 	uint64_t vsaOffset = 0;