Updated Qemu Guest Agent URL's to use the latest release by default

[yavuzaydin]

No description provided.

[ader1990]

Hello,

As these are critical services that offer a channel between host and guest, there has to be some sort of hash checks or pinning to version.

We should be able to follow the same approach with the virtio ISOs, and leave the end user to decide what it gets installed. Using the latest is a no-go for me, as one cannot know beforehand what latest means.

This change should be done in a rather iterative way, first have a way to customize the url and with checksum based checks.

I propose the following new config, leaving everything backwards compatible:

[custom]
install_qemu_ga = True

[virtio_qemu_guest_agent]
url = https://fedorapeople.org/groups/virt/virtio-win/direct-downloads/archive-qemu-ga/qemu-ga-win-100.0.0.0-3.el7ev/qemu-ga-x86.msi
checksum = <checksum-sha256>

if (custom.install_qemu_ga) {
  if !(virtio_qemu_guest_agent.url  && virtio_qemu_guest_agent.checksum) {
     # execute current behaviour
  } else {
     # execute new behaviour
  }
}

This way, everything can be customized and it is as the same time, safe.

In another patch, we can improve on this and have a new flag use_qemu_ga_from_virtio_drivers = True, where the corresponding version can be used.