qemu: Prevent serial logs from being truncated on revert

[richiejp]

QEMU overwrites the serial file by default. Because we now restart QEMU during
revert to snapshot, this results in the serial file being truncated. However
we can make use of the logfile and logappend chardev options to overcome this.

Verification run: http://10.160.67.78/tests/641

This is low priority.

[codecov]

Codecov Report

Merging #1002 into master will increase coverage by 0.1%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master    #1002     +/-   ##
=========================================
+ Coverage   40.27%   40.37%   +0.1%     
=========================================
  Files          34       34             
  Lines        4554     4555      +1     
  Branches      786      786             
=========================================
+ Hits         1834     1839      +5     
+ Misses       2388     2381      -7     
- Partials      332      335      +3

Impacted Files	Coverage Δ
backend/qemu.pm	31.14% <75%> (+0.12%)	⬆️
isotovideo	60.98% <0%> (+1.15%)	⬆️

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