EmulatorPkg: IoThunk->Close() is called too early, may causing hang

EmulatorPkg: IoThunk->Close() is called too early, may causing hang

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1121

To produce a EFI_BLOCK_IO_PROTOCOL instance in Emulator platform,
EmulatorPkg defines the EMU_IO_THUNK_PROTOCOL. OS dependent layer
needs to produce this protocol implementation and a generic OS
independent layer consumes this protocol to produce
EFI_BLOCK_IO_PROTOCOL.

EMU_IO_THUNK_PROTOCOL can also be used to abstract the OS dependent
IO operation for other UEFI protocols, e.g.: GOP, SimpleFileSystem
and etc.

It contains two interfaces Open() and Close(). Open() creates the
specific IO instances, e.g. for Block IO access, File System access,
Screen access, etc. Close() destroys the specific IO instances.

Later on the Emulator generic module (e.g.: EmuBlockIoDxe) calls
Open() to create the IO instance in DriverBindingStart() and calls
Close() in DriverBindingStop().
But today's implementation of DriverBindingStop() contains a bug
that it calls Close() before uninstalling the EFI_BLOCK_IO_PROTOCOL.

It's a mistake in code. Take EFI_BLOCK_IO for example,
the uninstallation may cause the upper layer driver that consumes
EFI_BLOCK_IO call BlockIo.Reset(), which consequently calls
EmuBlockIo.Reset(). But the EmuBlockIo instance is already destroyed
by Close() that happens before uninstallation.

So a proper implementation is to call Close() after uninstallation
succeeds.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1121

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Andrew Fish <afish@apple.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>