Mark NonDiscoverablePciDeviceIo Memory XP By Default (#374)

# Preface

Please ensure you have read the [contribution
docs](https://github.com/microsoft/mu/blob/master/CONTRIBUTING.md) prior
to submitting the pull request. In particular,
[pull request
guidelines](https://github.com/microsoft/mu/blob/master/CONTRIBUTING.md#pull-request-best-practices).

## Description

When allocating memory for a non-discoverable PCI device's IO, the
current core code removes the XP attribute, allowing code to execute
from that region. This is a security vulnerability and unneeded. This
change updates to mark the region as XP when allocating memory for the
non-discoverable PCI device.

These allocations in this function are limited to `EfiBootServicesData`
and `EfiRuntimeServicesData`, which we expect to be XP.

For each item, place an "x" in between `[` and `]` if true. Example:
`[x]`.
_(you can also check items in the GitHub UI)_

- [x] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
functions?
- Examples: Add a new library, publish a new PPI, update an algorithm,
...
- [x] Impacts security?
- **Security** - Does the change have a direct security impact on an
application,
    flow, or firmware?
  - Examples: Crypto algorithm change, buffer overflow fix, parameter
    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
break
    in build or boot behavior?
- Examples: Add a new library class, move a module to a different repo,
call
    a function in a new library class in a pre-existing module, ...
- [ ] Includes tests?
  - **Tests** - Does the change include any explicit test code?
  - Examples: Unit tests, integration tests, robot tests, ...
- [ ] Includes documentation?
- **Documentation** - Does the change contain explicit documentation
additions
    outside direct code modifications (and comments)?
- Examples: Update readme file, add feature readme file, link to
documentation
    on an a separate Web page, ...

## How This Was Tested

Tested on QEMU and a physical platform.

## Integration Instructions

N/A.

MdeModulePkg/Bus/Pci/NonDiscoverablePciDeviceDxe/NonDiscoverablePciDeviceIo.c

@@ -1181,7 +1181,7 @@ NonCoherentPciIoAllocateBuffer (
   Status = gDS->SetMemorySpaceAttributes (
                   (EFI_PHYSICAL_ADDRESS)(UINTN)AllocAddress,
                   EFI_PAGES_TO_SIZE (Pages),
-                  MemType
+                  MemType | EFI_MEMORY_XP // MU_CHANGE: Alloc MMIO memory XP by default
                   );
   if (EFI_ERROR (Status)) {
     goto RemoveList;