docs: Explain how to login into built vm

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nixos
doc
manual
installation
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nixos/doc/manual/installation/changing-config.xml
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</screen>
The VM does not have any data from your host system, so your existing
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user accounts and home directories will not be available. You can
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forward ports on the host to the guest. For instance, the following
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will forward host port 2222 to guest port 22 (SSH):
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user accounts and home directories will not be available unless you
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have set <literal>mutableUsers = false</literal>. Another way is to
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temporarily add the following to your configuration:
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<screen>
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users.extraUsers.your-user.initialPassword = "test"
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</screen>
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<emphasis>Important:</emphasis> delete the $hostname.qcow2 file if you
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have started the virtual machine at least once without the right
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users, otherwise the changes will not get picked up.
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You can forward ports on the host to the guest. For
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instance, the following will forward host port 2222 to guest port 22
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(SSH):
<screen>
$ QEMU_NET_OPTS="hostfwd=tcp::2222-:22" ./result/bin/run-*-vm