disk: use bytes instead of sectors in all code

Use bytes internally everywhere and convert to sectors only when writing
the options for the stages.

Changed the AlignUp() method to not do the alignment if the input is
already aligned.  This changes the behaviour when the size is 0, but
that's not a realistic use case.  Updated unit tests to match.

Manifests are unaffected.

Co-Authored-By: Christian Kellner <christian@kellner.me>

internal/disk/disk_test.go

@@ -12,20 +12,23 @@ import (
 func TestDisk_AlignUp(t *testing.T) {
 
 	pt := disk.PartitionTable{}
-	firstAligned := pt.BytesToSectors(disk.DefaultGrainBytes)
+	firstAligned := disk.DefaultGrainBytes
 
 	tests := []struct {
 		size uint64
 		want uint64
 	}{
-		{0, firstAligned},
+		{0, 0},
 		{1, firstAligned},
 		{firstAligned - 1, firstAligned},
+		{firstAligned, firstAligned}, // grain is already aligned => no change
+		{firstAligned / 2, firstAligned},
+		{firstAligned + 1, firstAligned * 2},
 	}
 
 	for _, tt := range tests {
 		got := pt.AlignUp(tt.size)
-		assert.Equal(t, tt.want, got)
+		assert.Equal(t, tt.want, got, "Expected %d, got %d", tt.want, got)
 	}
 }