hw/i386: add 4g boundary start to X86MachineState

Rather than hardcoding the 4G boundary everywhere, introduce a
X86MachineState field @above_4g_mem_start and use it
accordingly.

This is in preparation for relocating ram-above-4g to be
dynamically start at 1T on AMD platforms.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20220719170014.27028-2-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>