Add warning about differences from the shell when using qemuargs

[MalikKillian]

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Description

Add a warning about the differences between using QEMU arguments from a command line versus including them as an argument in Packer. Specifically, certain special characters like semi-colons require escaping when used on the command line. Packer passes arguments directly to the QEMU executable without involving the special character interpretation of the shell. Therefore, attempts to escape these characters as you would on the command line can actually result in a bad argument being passed.

A likely small, but very important, use-case for adding an additional QEMU argument is performing cloud-init by serving files from an HTTP server. The cloud-init docs directly mention this use-case and provide an example for starting QEMU.

https://cloudinit.readthedocs.io/en/latest/tutorial/qemu.html#launch-a-virtual-machine-with-our-user-data

It shows the -smbios argument as partially quoted to avoid the interpretation of the semi-colon as a break in the command. From a shell this is necessary but from Packer it will cause the argument to essentially be rejected, only silently. So it may be very hard to notice the error.