[AArch64] salt.control.virt support

[AArch64] salt.control.virt support

Salt relies on a limiting libvirt_domain j2 template to generate the
XML it passes to libvirt for salt.control managed virtual machines.
For AArch64, we need to set up 3 XML nodes in a non-default way:
1. UEFI firmware (AAVMF) should be enabled by passing a pflash loader;
2. CPU mode should be 'host-passthrough';
3. QEMU machine type should be 'virt';

To allow configuring the above using pillar data:
- virtng module: implement functionality similar to upstream changes:
  * 219b84a512 virt module: Allow NVRAM unlinking on DOM undefine
    in develop, not in 2018.2;
  * 9cace9adb9 Add support to virt for libvirt loader
    in develop, not in 2018.2;
- virtng module: extend it with:
  * pass virt machine type to vm;
  * pass cpu_mode to vm;

JIRA: ARMBAND-404

Change-Id: Ib2123e7170991b3dfbdb42bd1a2baa5a4360b200
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>