OvmfPkg/QemuFlashFvbServicesRuntimeDxe: eliminate unchecked PcdSetXX() calls

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: eliminate unchecked PcdSetXX() calls

These are deprecated / disabled under the
DISABLE_NEW_DEPRECATED_INTERFACES feature test macro.

Introduce a variable called PcdStatus, and use it to assert the success of
these operations (there is no reason for them to fail here).

Cc: Jordan Justen <jordan.l.justen@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=166
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>