pc: fail memory hot-plug/unplug with -no-acpi and Q35 machine type

When -no-acpi option is used with Q35 machine type, no guest ACPI is
built, but the ACPI device is still created, so only checking the
presence of ACPI device before memory plug/unplug is not enough in
such cases. Check whether ACPI is disabled globally in addition and
fail memory plug/unplug if it's disabled.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20171222015120.31730-1-haozhong.zhang@intel.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
