MSHV (Microsoft Hypervisor) doesn't expose an ITS to guests. Instead it
exposes a GICv2m MSI Frame for mapping MSIs to SPIs.
Update the MADT generation code to create the GIC MSI Frame structure
instead of the ITS structure whenever the guest vGIC is using a GICv2m
frame. This allows booting guests via firmware on MSHV.
Signed-off-by: Anirudh Rayabharam <anrayabh@microsoft.com>