hw/arm/virt-acpi-build: remove unnecessary variable mcfg_start

hw/arm/virt-acpi-build: remove unnecessary variable mcfg_start

mcfg_start points to the start of MCFG table and is used in
build_header. While this information could be derived from mcfg.

This patch removes the unnecessary variable mcfg_start.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190419003053.8260-3-richardw.yang@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>