utils: Move FC's function to revert bytes to utils

Firecracker's revertBytes function, now called "RevertBytes", can be
exposed as part of the virtcontainers' utils file, as this function will
be reused by Cloud Hypervisor, when adding the rate limiter logic there.

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

src/runtime/virtcontainers/utils/utils_test.go

@@ -569,3 +569,14 @@ func TestGetAllParentPaths(t *testing.T) {
 		assert.Equal(tc.parents, getAllParentPaths(tc.targetPath))
 	}
 }
+
+func TestRevertBytes(t *testing.T) {
+	assert := assert.New(t)
+
+	//10MB
+	testNum := uint64(10000000)
+	expectedNum := uint64(10485760)
+
+	num := RevertBytes(testNum)
+	assert.Equal(expectedNum, num)
+}