GDB: Correctly report stop reasons for watchpoints

[mkroening]

This fixes watchpoints in GDB (an oversight from #164).

Found while working on integration tests, @jounathaen. ;)

bors r+

[codecov]

Codecov Report

Merging #204 (7b5c1b1) into master (d8f008d) will increase coverage by 5.58%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   14.91%   20.49%   +5.58%     
==========================================
  Files          17       17              
  Lines        2635     2654      +19     
==========================================
+ Hits          393      544     +151     
+ Misses       2242     2110     -132     

Impacted Files	Coverage Δ
src/arch/x86_64/registers/debug.rs	0.00% <0.00%> (ø)
src/linux/gdb/mod.rs	0.00% <0.00%> (ø)
src/linux/uhyve.rs	54.80% <0.00%> (+6.73%)	⬆️
src/vm.rs	53.40% <0.00%> (+23.29%)	⬆️

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[bors]

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