hw/i386/acpi: Set PCAT_COMPAT bit only when pic is not disabled

hw/i386/acpi: Set PCAT_COMPAT bit only when pic is not disabled

A value 1 of PCAT_COMPAT (bit 0) of MADT.Flags indicates that the system
also has a PC-AT-compatible dual-8259 setup, i.e., the PIC.  When PIC
is not enabled (pic=off) for x86 machine, the PCAT_COMPAT bit needs to
be cleared.  The PIC probe should then print:

   [    0.155970] Using NULL legacy PIC

However, no such log printed in guest kernel unless PCAT_COMPAT is
cleared.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-ID: <20240403145953.3082491-1-xiaoyao.li@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>