OvmfPkg: set SMM stack size to 16KB

OvmfPkg: set SMM stack size to 16KB

The default stack size (from UefiCpuPkg/UefiCpuPkg.dec) is 8KB, which
proved too small (i.e., led to stack overflow) across commit range
98c2d9610506^..f85d3ce2efc2^, during certificate enrollment into "db".

As the edk2 codebase progresses and OVMF keeps including features, the
stack demand constantly fluctuates; double the SMM stack size for good
measure.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Ref: http://thread.gmane.org/gmane.comp.bios.edk2.devel/12864
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1341733
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>