ArmVirtPkg: use protocol-based DevicePathLib instance for most DXE modules

ArmVirtPkg: use protocol-based DevicePathLib instance for most DXE modules

Port OvmfPkg commit 5c3481b0b611e to ArmVirtPkg. Some explanation should
be in order (because 5c3481b0b611e doesn't offer any):

- The UefiDevicePathLibDevicePathProtocol instance uses the Device Path
  Utilities Protocol, produced by DevicePathDxe, for formatting and
  parsing the textual device path representation. This allows for a
  lighter weight lib instance that gets linked into several DXE modules.
  In comparison, the more standalone UefiDevicePathLib instance includes
  the formatting and parsing routines in every client module.

- The DXE core needs DevicePathLib before it dispatches DevicePathDxe, so
  it needs to stick with the standalone instance.

- DevicePathDxe itself also needs the standalone instance, for
  implementing the protocol.

- The DXE-phase PCD driver, "MdeModulePkg/Universal/PCD/Dxe/Pcd.inf",
  depends on DevicePathLib via UefiLib and DxeServicesLib at the least; so
  with this update, it inherits a dependency on the protocol. In reverse,
  DevicePathDxe depends on the PCD Protocol, via PcdLib. The cycle is
  broken by using BasePcdLibNull in DevicePathDxe. That restricts it to
  FixedAtBuild, Patch, and FeatureFlag PCDs, but that's fine.

Example space savings (using ArmVirtQemu and the GCC5 toolchain):
- NOOPT:   187KB in FVMAIN, 12KB in FVMAIN_COMPACT
- DEBUG:   147KB in FVMAIN, 20KB in FVMAIN_COMPACT
- RELEASE: 123KB in FVMAIN, 17KB in FVMAIN_COMPACT

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=940
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>