OvmfPkg/PeilessStartupLib: move mPageTablePool to stack

OvmfPkg/PeilessStartupLib: move mPageTablePool to stack

PeilessStartupLib is running in SEC phase. In this phase global variable
is not allowed to be modified. This patch moves mPageTablePool to stack
and pass it as input parameter between functions.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>