[Feature] Add remote target support for QEMU builder

[ghost]

This issue was originally opened by @stdevel as hashicorp/packer#7757. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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As far as I understood from the documentation, the QEMU builder does not support remote hosts running KVM/QEMU.
This means, you will need to run Packer on that host that also runs KVM.
It would be very useful to have the possibility to specify a hypervisor host and have Packer to connect to it via SSH (like the vsphere-iso builder).

[mdebord-dlr]

Adding here since this is a big priority in my use-case.

From what I can tell, currently the hypervisor must be used as the primary worker node for qemu with no way of orchestrating Packer from a CI/CD worker node or any other worker host. Everything must be done on the hypervisor locally which really is a hindrance to system/workflow design.

The community "libvirt" plugin does allow remote access (although it plays a different role versus qemu plugin, so it isn't a direct replacement). For example, it uses a "qemu://host.example.com/system" style connection string. Perhaps that same method of connecting could be adapted here?