OvmfPkg/PciHotPlugInitDxe: generalize RESOURCE_PADDING composition

PciHotPlugInitDxe has a static variable called "mPadding" (of type
RESOURCE_PADDING), which describes two constant resource reservations:

- MmioPadding: 2MB of non-prefetchable (hence 32-bit) MMIO space,

- IoPadding: 512B of IO space.

In the GetResourcePadding() member function of
EFI_PCI_HOT_PLUG_INIT_PROTOCOL, the driver outputs a dynamically allocated
verbatim copy of "mPadding", for PciBusDxe to consume in its
ApplyResourcePadding() function.

In a later patch, we're going to compose the set of resource reservations
dynamically, based on QEMU hints. Generalize the RESOURCE_PADDING
structure so that we may generate (or not generate) each resource type
individually:

- Replace the named "MmioPadding" and "IoPadding" fields in
  RESOURCE_PADDING with an array of descriptors,

- remove "mPadding",

- in GetResourcePadding(), request the same (default) reservations as
  before, as if we attempted and failed to fetch the QEMU hints.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Marcel Apfelbaum <marcel@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/PciHotPlugInitDxe/PciHotPlugInit.inf

@@ -29,6 +29,7 @@
   MdePkg/MdePkg.dec
 
 [LibraryClasses]
+  BaseMemoryLib
   DebugLib
   DevicePathLib
   MemoryAllocationLib