Introduce memory types RAM and DEVICE, along with a way for subsystems to
query the global memory banks. This is required by VFIO, which will need
to pin and map guest RAM so that assigned devices can safely do DMA to it.
Depending on the architecture, the physical map is made of either one or
two RAM regions. In addition, this new memory types API paves the way to
reserved memory regions introduced in a subsequent patch.

For the moment we put vesa and ivshmem memory into the DEVICE category, so
they don't have to be pinned. This means that physical devices assigned
with VFIO won't be able to DMA to the vesa frame buffer or ivshmem. In
order to do that, simply changing the type to "RAM" would work. But to
keep the types consistent, it would be better to introduce flags such as
KVM_MEM_TYPE_DMA that would complement both RAM and DEVICE type. We could
then reuse the API for generating firmware information (that is, for x86
bios; DT supports reserved-memory descripti...