Expand ARMv8 MMIO32 for 256 MiB VMSVGA VRAM

[moreaki]

Summary

Expand the ARMv8 PCI 32-bit MMIO aperture from 256 MiB to 448 MiB.

This allows a VMSVGA configuration with 256 MiB VRAM to keep both required BARs in low PCI MMIO space:

• VMSVGA VRAM BAR: 256 MiB, prefetchable
• VMSVGA3 MMIO BAR: 4 MiB

Why

On the tested macOS/Arm Windows 11 ARM setup, 128 MiB VRAM was marginal for true 5760x3240 Retina operation. Increasing VRAM to 256 MiB failed unless enough ARMv8 low PCI MMIO space was available for both VMSVGA BARs.

Moving the VRAM BAR to 64-bit MMIO was not sufficient locally; the guest booted only in a degraded display state with the Windows driver not initializing normally.

Validation

• Applied independently to origin/main.
• git diff --check passes.
• Rebuilt locally as part of the macOS/Arm test build.
• Runtime verified with vram=256, 3D enabled, Windows 11 ARM guest mode 5760x3240x32, and Guest Additions run level 3.

Observed layout:

• ARM pci-mmio32: 0xe0000000..0xfbffffff
• VMSVGA VRAM BAR: 0xe0000000..0xefffffff
• VMSVGA3 MMIO BAR: 0xf0400000..0xf07fffff

Refs #742.

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