OvmfPkg: add SecureBootVariableLib class resolution

OvmfPkg: add SecureBootVariableLib class resolution

The edk2 patch
  SecurityPkg: Create library for setting Secure Boot variables.

moves generic functions from SecureBootConfigDxe and places
them into SecureBootVariableLib. This patch adds SecureBootVariableLib
mapping for OvmfPkg.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Sunny Wang <sunny.wang@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>