core: Print info message if provisioning is skipped by the sentinel file

Tell the user if the VM has already been provisioned and no
`--[no-]provision` option has been specified. This should reduce the
confusion of the 1.3+ functionality.
Run "ip link set [eth1] down" before running "netctl start [eth1]" in Arch guest network configuration
Add the symlink back that disabled renaming of network interface names
Merge pull request #2601 from mattalexx/fix-issue-2599

guests/arch: run "ip link set [eth1] down" before running "netctl start [eth1]"
Merge pull request #2600 from tmatilai/info-if-provisioned

core: print info message if provisioning is skipped by the sentinel file
core: style nitpicks
core: improve guest type detection

/cc @fgrehm - I figured you might be interested in this. :)