OvmfPkg/QemuFwCfgLib: move InternalQemuFwCfgIsAvailable() to lib instances

OvmfPkg/QemuFwCfgLib: move InternalQemuFwCfgIsAvailable() to lib instances

InternalQemuFwCfgIsAvailable() is an API that is incorrectly exposed by
the "OvmfPkg/Include/Library/QemuFwCfgLib.h" library class header; the API
is meant to be used internally to library instances (if it's needed at
all).

In OvmfPkg, we have two lib instances (for SEC and PEI/DXE); they provide
different implementations of InternalQemuFwCfgIsAvailable(), for the
shared file "OvmfPkg/Library/QemuFwCfgLib/QemuFwCfgLib.c". Move the API
declaration to a new internal header called "QemuFwCfgLibInternal.h", and
drop EFIAPI in the process.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>