ArmVirtPkg: introduce new ArmQemuRelocatablePlatformLib

This introduces ArmQemuRelocatablePlatformLib, which started out as a
straight copy of ArmXenRelocatablePlatformLib, but has been modified so
that ArmVirtPkg/PrePi/ArmVirtPrePiUniCoreRelocatable.inf can be used with
QEMU as well as with Xen. It retains the self relocation and FDT parsing
for the system memory, but uses the QEMU MMU layout.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>