OvmfPkg: Disable variable runtime cache

OvmfPkg: Disable variable runtime cache

Sets gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache
to FALSE in OvmfPkgIa32.dsc, OvmfPkgIa32X64.dsc, and OvmfPkgX64.dsc
so that when SMM_REQUIRE is TRUE, the SMM variable driver will not
use the runtime variable cache.

This is done for OvmfPkg because it currently depends upon a SMM
variable GetVariable ()implementation as a simple method to exercise
the SMM driver stack. This allows the following commands to be used
for variables such as Boot####, BootOrder, and BootNext to test SMM
timing and stability differences on the BSP (e.g. CPU#0) vs an
AP (e.g. CPU#1).
 # taskset -c 0 efibootmgr
 # taskset -c 1 efibootmgr

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>