OvmfPkg/PlatformPei: add missing auto variable initialization

OvmfPkg/PlatformPei: add missing auto variable initialization

The E820EntriesCount variable in XenPublishRamRegions() may be
referenced without being initialized on RELEASE builds, since the
ASSERT that fires if the call to XenGetE820Map() fails is compiled
out in that case. So initialize it to 0.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>