drm/i915/gvt: add KVMGT support

KVMGT is the MPT implementation based on VFIO/KVM. It provides
a kvmgt_mpt ops to gvt for vGPU access mediation, e.g. to
mediate and emulate the MMIO accesses, to inject interrupts
to vGPU user, to intercept the GTT writing and replace it with
DMA-able address, to write-protect guest PPGTT table for
shadowing synchronization, etc. This patch provides the MPT
implementation for GVT, not yet functional due to theabsence
of mdev.

It's built as kvmgt.ko, depends on vfio.ko, kvm.ko and mdev.ko,
and being required by i915.ko. To not introduce hard dependency
in i915.ko, we used indirect symbol reference. But that means
users have to include kvmgt.ko into init ramdisk if their
i915.ko is included.

Signed-off-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Xiaoguang Chen <xiaoguang.chen@intel.com>
Signed-off-by: Jike Song <jike.song@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>