x86: Check for machine state object class before typecasting it

x86: Check for machine state object class before typecasting it

In ed9e923c3c ("x86: move SMM property to X86MachineState", 2019-12-17)
In v4.2.0-246-ged9e923c3c the SMM property was moved from PC
machine class to x86 machine class. Makes sense, but the change
was too aggressive: in target/i386/kvm.c:kvm_arch_init() it
altered check which sets SMRAM if given machine has SMM enabled.
The line that detects whether given machine object is class of
PC_MACHINE was removed from the check. This makes qemu try to
enable SMRAM for all machine types, which is not what we want.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Fixes: ed9e923c3c ("x86: move SMM property to X86MachineState", 2019-12-17)
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <7cc91bab3191bfd7e071bdd3fdf7fe2a2991deb0.1577692822.git.mprivozn@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>