OvmfPkg: Use user-specified opt/ovmf/X-PciMmio64Mb value unconditionally

Prior to this change, OVMF considers opt/ovmf/X-PciMmio64Mb the
minimum aperture size, allowing us to force the window to be larger
but not smaller than what PlatformDynamicMmioWindow calculates.

Adjust OVMF so that a smaller value for the aperture is honored.

Context:
Due to an inefficiency in the way older host kernels manage
pfnmaps for guest VM memory ranges [0], guests with large-BAR
GPUs passed-through have a very long (multiple minutes) initialization
time when the MMIO window advertised by OVMF is sufficiently sized for
the passed-through BARs (i.e., the correct OVMF behavior). However, on
older distro series such as Ubuntu Jammy, users have benefited from fast
guest boot times when OVMF advertised an MMIO window that was too small
to accommodate the full BAR, since this resulted in the long PCI initialization
process being skipped (and retried later, if pci=realloc pci=nocrs were set).

While the root cause is being fully addressed in the upstream kernel [1],
the solution relies on huge pfnmap support, which is a substantial series
with many ABI changes that is unlikely to land in many LTS and legacy distro kernels,
including those of Ubuntu Noble. As a result, the only kernel improvement
supported on those kernels is this patch [2], which reduces the extra boot
time by about half. Unfortunately, that boot time is still an average of
1-3 minutes longer per-VM-boot than what can be achieved when the host is
running a version of OVMF without PlatformDynamicMmioWindow (PDMW) support
(introduced in [3])

Since there is no way to force the use of the classic MMIO window size[4]
in any version of OVMF after [3], and since we have a use case for such
functionality on legacy distro kernels that would yield significant,
recurring compute time savings across all impacted VMs, this change to
this knob's behavior seems appropriate.

[0]: https://lore.kernel.org/all/CAHTA-uYp07FgM6T1OZQKqAdSA5JrZo0ReNEyZgQZub4mDRrV5w@mail.gmail.com/
[1]: https://lore.kernel.org/all/20250205231728.2527186-1-alex.williamson@redhat.com/
[2]: https://lore.kernel.org/all/20250111210652.402845-1-alex.williamson@redhat.com/
[3]: ecb778d
[4]: https://edk2.groups.io/g/devel/topic/109651206?p=Created,,,20,1,0,0

Signed-off-by: Mitchell Augustin <mitchell.augustin@canonical.com>

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