Add test for lvm.ExtendLV.

[smoser]

Another system test to push test through the creation of an LVM, and subsequent extention of it.

So, things to point out:

• I think that the 'cleanups' mechanism works. I'm sure there is something simpler or less verbose. Basically I just want to ast.Fail a test if the cleanup fails.
• The function is very long, and makes me feel like the api could use some work.

[codecov[bot]]

Codecov Report

Merging #87 into master will increase coverage by 6.65%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   59.57%   66.23%   +6.65%     
==========================================
  Files          16       16              
  Lines        1937     1937              
==========================================
+ Hits         1154     1283     +129     
+ Misses        654      487     -167     
- Partials      129      167      +38     

Impacted Files	Coverage Δ
linux/disk.go	56.59% <0.00%> (+0.89%)	:arrow_up:
linux/util.go	79.69% <0.00%> (+4.51%)	:arrow_up:
linux/lvmdump.go	73.56% <0.00%> (+17.24%)	:arrow_up:
linux/lvm.go	49.21% <0.00%> (+40.94%)	:arrow_up:

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