Fix consensus

[wangchao0222]

Description

不允许从pow共识升级为其他共识

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Brief of your solution

升级共识时，如果当前共识时pow则直接返回升级失败

How Has This Been Tested?

当前共识为pow共识，升级共识会显示失败

[codecov-commenter]

Codecov Report

Merging #316 (7b094eb) into master (0f65698) will decrease coverage by 0.04%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
- Coverage   45.83%   45.79%   -0.05%     
==========================================
  Files         138      138              
  Lines       12267    12272       +5     
==========================================
- Hits         5623     5620       -3     
- Misses       5491     5498       +7     
- Partials     1153     1154       +1     

Impacted Files	Coverage Δ
kernel/consensus/pluggable_consensus.go	17.85% <0.00%> (-0.37%)	:arrow_down:
bcs/contract/xvm/code_manager.go	63.63% <0.00%> (-2.73%)	:arrow_down:
bcs/consensus/xpoa/xpoa.go	51.26% <0.00%> (-2.04%)	:arrow_down:
bcs/consensus/tdpos/schedule.go	43.57% <0.00%> (+0.91%)	:arrow_up:
bcs/network/p2pv2/server.go	52.40% <0.00%> (+1.06%)	:arrow_up:

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