target/i386/sev: Restrict SEV to system emulation

target/i386/sev: Restrict SEV to system emulation

SEV is irrelevant on user emulation, so restrict it to sysemu.
Some stubs are still required because used in cpu.c by
x86_register_cpudef_types(), so move the sysemu specific stubs
to sev-sysemu-stub.c instead. This will allow us to simplify
monitor.c (which is not available in user emulation) in the
next commit.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211007161716.453984-14-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>