ArmVirtPkg/ArmVirtXen: add ArmVirtMemInfoLib implementation

ArmVirtPkg/ArmVirtXen: add ArmVirtMemInfoLib implementation

Clone the existing ArmPlatformGetVirtualMemoryMap () for this platform,
clean it up slightly (by using a static buffer rather than a heap
allocation, and removing the support for uncached DRAM mappings), and
turn it into a new ArmVirtMemInfoLib implementation.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Julien Grall <julien.grall@linaro.org>