OvmfPkg/VirtioNetDxe: document Rx/Tx allocs and mappings on Init/Shutdown
    
    Document the following actions on the
    
      EfiSimpleNetworkStarted <-> EfiSimpleNetworkInitialized
    
    state transitions:
    
    * from commit 46b11f00ac70 ("OvmfPkg/VirtioNetDxe: alloc RxBuf using
      AllocateSharedPages()", 2017-09-14):
    
      VirtioNetInitRx ->
      { VirtIo->AllocateSharedPages, VirtioMapAllBytesInSharedBuffer }
    
      VirtioNetShutdownRx ->
      { VirtIo->UnmapSharedBuffer, VirtIo->FreeSharedPages }
    
    * from commit 891f016c1b99 ("OvmfPkg/VirtioNetDxe: dynamically alloc
      transmit header", 2017-09-14):
    
      VirtioNetInitTx ->
      { VirtIo->AllocateSharedPages, VirtioMapAllBytesInSharedBuffer }
    
      VirtioNetShutdownTx ->
      { VirtIo->UnmapSharedBuffer, VirtIo->FreeSharedPages }
    
    Cc: Brijesh Singh <brijesh.singh@amd.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>