ArmVirtPkg/QemuVirtMemInfoLib: use HOB not PCD to record the memory size

ArmVirtPkg/QemuVirtMemInfoLib: use HOB not PCD to record the memory size

Due to the way we inherited the formerly fixed PCDs to describe the
system memory base and size from ArmPlatformPkg, we ended up with a
MemoryInit PEIM that relies on dynamic PCDs to communicate the size of
system memory between the constructor of one of its library dependencies
and the core module. This is unnecessary, and forces us to incorporate
the PCD PEIM as well, for no good reason. So instead, let's use a HOB.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>