[qemu] pass --conf-file to dnsmasq (Fixes #1468)

[Saviq]

Otherwise it may try (and fail) to read /etc/dnsmasq.conf, when it exists, and die.

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To test:

$ sudo touch /etc/dnsmasq.conf
$ snap restart multipass
$ systemctl status snap.multipass.multipassd.service
# ensure dnsmasq is running under CGroup
$ sudo rm /etc/dnsmasq.conf

[multipass-ci-bot]

macOS build available: multipass-1.2.0-dev.141.pr1471+g0dc0ffb9.mac-Darwin.pkg
Snap build available: snap refresh multipass --channel edge/pr1471

[codecov]

Codecov Report

Merging #1471 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1471      +/-   ##
==========================================
+ Coverage   73.58%   73.59%   +0.01%     
==========================================
  Files         210      210              
  Lines        7616     7620       +4     
==========================================
+ Hits         5604     5608       +4     
  Misses       2012     2012              

Impacted Files	Coverage Δ
src/platform/backends/qemu/dnsmasq_process_spec.h	100.00% <ø> (ø)
...rc/platform/backends/qemu/dnsmasq_process_spec.cpp	93.93% <100.00%> (+0.39%)	⬆️
src/platform/backends/qemu/dnsmasq_server.cpp	92.53% <100.00%> (+0.22%)	⬆️

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[townsend2010]

Tried edge version of multipass with /etc/dnsmasq.conf in place and verified dnsmasq will not start. Then refreshed to this PR's version and verified dnsmasq is indeed running.

+1

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push