Allow VNC passwords to be set from config

[carlosonunez]

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Description

As a person troubleshooting machines provisioned by the QEMU builder
I want the ability to set a VNC password
So that I don't have to enable debug logging to get the password
And I can troubleshoot issues more quickly.

Use Case(s)

• Faster, easier troubleshooting during config iteration

Potential configuration

source "qemu" "machine" {
 # rest of config
 vnc_use_password = true
 vnc_password = "supersecret"
 # rest of config
}

Potential References

• Pull request: link