Slicing with xarray backend does not work if axis is reversed

[drs251]

This one is quite subtle: everything works nicely, until you try to slice a Curve or an Image:

import xarray as xr
import holoviews as hv
xs = np.arange(30)[::-1]
ys = np.random.rand(30)
array = xr.DataArray(ys, coords=[('x', xs)], name='y')
hv.Dataset(array).to(hv.Curve)[5:15]

Removing [::-1] solves the problem, but when working with real xarray objects, this is not so easy sometimes, so it would be great to fix this.

(This was tested on version 1.10.2)