Release the zocl call by calling `device.close()`

[mariodruiz]

• Release the zocl call by calling device.close()
• keep pl_server alive
• grab XRT handle in _xrt_download if the handle is empty

With these changes after a Overlay.free() the instance of zocl is released, and by not stopping the pl_server we can download another overlay from the same notebook.

[codecov-commenter]

Codecov Report

Merging #1338 (d67700d) into image_v2.8 (b85f453) will decrease coverage by 0.02%. The diff coverage is 14.28%.

@@              Coverage Diff               @@
##           image_v2.8    #1338      +/-   ##
==============================================
- Coverage       46.47%   46.45%   -0.03%     
==============================================
  Files              28       28              
  Lines            4953     4958       +5     
  Branches          971      972       +1     
==============================================
+ Hits             2302     2303       +1     
- Misses           2594     2598       +4     
  Partials           57       57              

Impacted Files	Coverage Δ
pynq/overlay.py	33.55% <0.00%> (-0.08%)	:arrow_down:
pynq/pl_server/xrt_device.py	29.13% <16.66%> (-0.05%)	:arrow_down:

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[mariodruiz]

Converting to draft to do more checking after this https://github.com/Xilinx/Kria-PYNQ/commit/92da27656d00c2b8843986f30590c4c832b80411