Implement network detection for static IPs

[johscheuer]

An idea how to fix #759 to enable the network detection (we still need to solve the xml generation for the host/dhcp entry).

I moved TestAccLibvirtDomain_Consoles into a separate file with linux only otherwise I was not able to run the tests locally, since syscall.TIOCGPTN is not supported on MacOS.

[MalloZup]

looks good . great work

[MalloZup]

@johscheuer can you check travis? should be a minor lint

go run golang.org/x/lint/golint -set_exit_status ./libvirt .

libvirt/resource_libvirt_domain_test_linux.go:33:1: exported function TestAccLibvirtDomain_Consoles should have comment or be unexported

libvirt/resource_libvirt_domain_test_linux.go:33:6: don't use underscores in Go names; func TestAccLibvirtDomain_Consoles should be TestAccLibvirtDomainConsoles

Found 2 lint suggestions; failing.

exit status 1

Makefile:42: recipe for target 'lint-check' failed

make: *** [lint-check] Error 1

The command "make lint-check" exited with 2.```

[johscheuer]

The lint issue should be fixed now. The funny thing is that when I run make lint-check golint doesn't complain 🤔