ArmVirtPkg: reduce preallocation of boot services data pages

ArmVirtPkg: reduce preallocation of boot services data pages

Commit SVN r18503 ("MdeModulePkg DxeCore: Take the range in resource
HOB for PHIT as higher priority") changed the GCD init logic to take
the sum of the region sizes in gMemoryTypeInformation[] into account
when estimating the minimal amount of memory needed to boot the system.

Unfortunately, as an unintended side effect, this change results in boot
failures of ArmVirtQemu when running with QEMU's default memory size of
128 MB. The reason is that the sum of the gMemoryTypeInformation region
sizes plus the size of the inital PEI region exceeds 128 MB. Since we do
not actually need to preallocate 20,000 pages' worth of BootServicesData
memory, reduce this figure to the more reasonable 12,000.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18533 6f19259b-4bc3-4df7-8a09-765794883524