OvmfPkg/VirtioGpuDxe: introduce with Component Name 2 and Driver Binding

This patch adds the skeleton of the driver: it implements the Component
Name 2 Protocol and the Driver  Binding Protocol, in accordance with the
generic and GOP-specific requirements set forth in the UEFI spec and the
Driver Writers' Guide.

The basic idea is that VGPU_DEV abstracts the virtio GPU device, while the
single VGPU_GOP that we intend to support at this point stands for "head"
(aka "scanout") #0.

For now, the Virtio Device Protocol is only used for driver binding; no
actual virtio operations are done yet. Similarly, we use a "dummy" GOP
GUID and protocol structure (a plain UINT8 object) for now, so that
GOP-consuming drivers don't look at what we produce just yet.

The driver is a bit different from the other virtio device drivers written
thus far:

- It implements the GetControllerName() member of the Component Name 2
  Protocol. (Formatting helpful names is recommended by UEFI.) As a "best
  effort", we format the PCI BDF into the name (a PCI backend is not
  guaranteed by VIRTIO_DEVICE_PROTOCOL). It should provide a more friendly
  experience in the shell and elsewhere.

- This driver seeks to support all RemainingDevicePath cases:
  - NULL: produce all (= one) child handles (= VGPU_GOP heads) at once,
  - End of Device Path Node: produce no child handles,
  - specific ACPI ADR Node: check if it's supportable, and produce it
    (only one specific child controller is supported).
  This is one of the reasons for separating VGPU_GOP from VGPU_DEV.

The driver is a hybrid driver: it produces both child handles (one, to be
exact), but also installs a structure (VGPU_DEV) directly on the VirtIo
controller handle, using gEfiCallerIdGuid as protocol GUID. This is a
trick I've seen elsewhere in edk2 (for example, TerminalDxe), and it is
necessary for the following reason:

In EFI_COMPONENT_NAME2_PROTOCOL.GetControllerName(), we must be able to
"cast down" a VirtIo ControllerHandle to our own private data structure
(VGPU_DEV). That's only possible if we install the structure directly on
the VirtIo ControllerHandle (thereby rendering the driver a hybrid
driver), because a child controller with our GOP implementation on it may
not exist / be passed in there.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://tianocore.acgmultimedia.com/show_bug.cgi?id=66
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

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