Printer preflight

[sssoleileraaa]

Description

• Added start-vm method for making sure the Export VM is running from external VMs. This is for both printing and transferring documents to a drive
• Added printer-preflight method for making sure only one supported printer is connected
• Added method to check if less than or more than one supported printer is connected and new error code to tell the client when more than one printer is connected: ERROR_MULTIPLE_PRINTERS_FOUND.

Test

1. Build the export debian package from this branch and install it in sd-export-usb
2. Run the client and see that you get the correct screens for each case: 0 printers connected, 1 printer connected, and multiple printers connected.

[sssoleileraaa]

i think the build-buster test failing is expected for now but i can't remember why... will look into this tomorrow

[sssoleileraaa]

@emkll - I see that this causes a regression where it's sending ERROR_GENERIC back when no printer is connected instead of PRINTER_NOT_FOUND so please hold off on reviewing until I switch it from Draft to Ready for review. Mostly I was wondering if you agree with how I added start-vm as a supported export method. I thought it made most sense to not send a status code so the client could see if that status is None which would mean that no errors occurred, but let me know if you disagree with this approach. Thanks!

[sssoleileraaa]

the build test is failing due to the following error reported freedomofpress/securedrop-client#699

the error message is:

Error downloading object: localwheels/Mako-1.0.7-py3-none-any.whl (87ee3f7): Smudge error: Error downloading localwheels/Mako-1.0.7-py3-none-any.whl (87ee3f74ba3ea544e683a5a22e7e34f4d1cf3ad34414b5f3858becf00facf1d6): batch response: Invalid deploy key for freedomofpress/securedrop-debian-packaging
Errors logged to /home/circleci/packaging/securedrop-debian-packaging/.git/lfs/logs/20200115T214038.961785472.log
Use `git lfs logs last` to view the log.
error: external filter 'git-lfs filter-process' failed
fatal: localwheels/Mako-1.0.7-py3-none-any.whl: smudge filter lfs failed
fetch: Fetching reference refs/heads/master
batch response: Invalid deploy key for freedomofpress/securedrop-debian-packaging
error: failed to fetch some objects from 'https://github.com/freedomofpress/securedrop-debian-packaging.git/info/lfs'
make: *** [Makefile:36: install-deps] Error 2
./scripts/update-requirements
gpg: key F08893B959CAB065: 6 signatures not checked due to missing keys

[sssoleileraaa]

The failing test might work after this fix: #50

Also, the latest commit was to fix rebase errors from export refactor.

[redshiftzero]

diff looks good and I manually tested the no printer case and the printing happy path. For multiple printers, I only have one supported printer to test, however based on visual review of the diff this logic lgtm.