OvmfPkg/EmuVariableFvbRuntimeDxe: stop using PcdVariableStoreSize

OvmfPkg/EmuVariableFvbRuntimeDxe: stop using PcdVariableStoreSize

In commit 62f43f7c1947c, we set PcdVariableStoreSize to the same value as
PcdFlashNvStorageVariableSize (which in turn comes from VARS_LIVE_SIZE in
"OvmfPkg.fdf.inc").

This equality between both PCDs is a false requirement from
EmuVariableFvbRuntimeDxe. FVB drivers should use
PcdFlashNvStorageVariableSize for supporting non-volatile variables (even
if they happen to be kept in RAM only), along the other PcdFlashNvStorage*
PCDs. Whereas PcdVariableStoreSize is for variables that are volatile at
the gRT->SetVariable() / gRT->GetVariable() API level.

(PlatformPei too bases the preallocation for EmuVariableFvbRuntimeDxe on
PcdFlashNvStorageFtwSpareSize.)

Replace PcdVariableStoreSize in EmuVariableFvbRuntimeDxe with the
same-value PcdFlashNvStorageVariableSize. This means no change in
behavior, and it allows us to increase PcdVariableStoreSize in a later
patch without disturbing EmuVariableFvbRuntimeDxe (or PlatformPei).

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Gary Lin <glin@suse.com>
Tested-by: Gary Lin <glin@suse.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>