OvmfPkg: QemuFlashFvbServicesRuntimeDxe: split out runtime DXE specifics

OvmfPkg: QemuFlashFvbServicesRuntimeDxe: split out runtime DXE specifics

In preparation for introducing an SMM interface to this driver, move the
following traits to separate files, so that we can replace them in the new
SMM INF file:

- Protocol installations. The SMM driver will install protocol interfaces
  in the SMM protocol database, using SMM services.

- Virtual address change handler and pointer conversions. SMM drivers run
  with physical mappings and pointers must not be converted.

There are further restrictions and changes for an SMM driver, but the rest
of the code either complies with those already, or will handle the changes
transparently. For example:

- SMM drivers have access to both UEFI and SMM protocols in their entry
  points (see the PI spec 1.4, "1.7 SMM Driver Initialization"),

- MemoryAllocationLib has an SMM instance that serves allocation requests
  with the gSmst->SmmAllocatePool() service transparently, allocating
  runtime-marked SMRAM.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18671 6f19259b-4bc3-4df7-8a09-765794883524