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.