Setting video_type to none erroneously still adds a VRAM parameter to the video model tag

[origintopleft]

Describe the bug
I'm working on a project that doesn't require the domain to have a video display, so I've chosen to disable it:

  config.vm.provider :libvirt do |virt|
    virt.title = "title"
    virt.description = "blah"
    virt.cpus = 1
    virt.memory = 1024

    virt.video_type = "none"
  end

This throws the following XML diff as a warning:

==> default: Libvirt failed to fully update the domain with the specified XML. Result differs from requested:
==> default: --- requested
==> default: +++ result
==> default:      </graphics>
==> default:      <audio id="1" type="none" />
==> default:      <video>
==> default: -      <model type="none" vram="256" />
==> default: +      <model type="none" />
==> default:      </video>
==> default:      <memballoon model="virtio">
==> default:        <address bus="0x00" domain="0x0000" function="0x0" slot="0x03" type="pci" />

libvirt seems to do the sensible thing and just drop the VRAM parameter from the generated XML, which allows vagrant up to continue as normal. Still, it's a big yellow block of text that told me to come report a bug, so here I am :V

Versions (please complete the following information)::

• Libvirt version: 10.0.0
• Vagrant version [output of vagrant version]: 2.4.0
• Vagrant flavour [Upstream or Distro]: Distro (Gentoo Linux, main Portage tree)
• Vagrant plugins versions (including vagrant-libvirt) [output of vagrant plugin list]:
  • vagrant-hostmanager (1.8.10, global)
  • vagrant-libvirt (0.12.2, global)