OvmfPkg/VirtioMmioDeviceLib: Implement VIRTIO_DEVICE_PROTOCOL for VirtIo Devices over MMIO

Why is the virtio-mmio implementation of the protocol a library,
instead of a driver binary?
The UEFI driver model would encourage to create a virtio-mmio driver
instead of a library. But the reasons why I created a library are:

- A virtio-mmio driver would imply an additional protocol that would
probably have a single attribute field:

typedef struct {
  PHYSICAL_ADDRESS       BaseAddress;
} VIRTIO_MMIO_DEVICE_PROTOCOL;

- There is no (easy) way to scan the available VirtIo devices on a
platform. So, the UEFI firmware for this platform would need a driver
to produce instances for every virtio devices it wants to expose in
UEFI. A single call to a helper library (ie: VirtioMmioDeviceLib)
make the porting easier.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Olivier Martin <olivier.martin@arm.com>

v5:
- typo fix in VirtioMmioInstallDevice() comment block
- plug MmioDevice leak in VirtioMmioUninstallDevice()
- return EFI_INVALID_PARAMETER in VirtioMmioGetQueueAddress() if
  QueueAddress is NULL
- VirtioMmioSetQueueSize(): fix return value (it's a status code)
- VirtioMmioSetPageSize(): check against EFI_PAGE_SIZE with "if" plus
  EFI_UNSUPPORTED, rather than ASSERT()
- VirtioMmioDeviceWrite(), VirtioMmioDeviceRead(): remove redundant
  (FieldSize > 8) checks
- VirtioMmioDeviceLib.inf: drop UefiDriverEntryPoint library dependency

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14965 6f19259b-4bc3-4df7-8a09-765794883524

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