ipmi: do not take/drop iothread lock

This is not necessary and actually causes a hang; it was probably copied
and pasted from KVM code, that is one of the very few places that run
outside iothread lock.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/ipmi/ipmi.c

@@ -51,9 +51,7 @@ static int ipmi_do_hw_op(IPMIInterface *s, enum ipmi_op op, int checkonly)
         if (checkonly) {
             return 0;
         }
-        qemu_mutex_lock_iothread();
         qmp_inject_nmi(NULL);
-        qemu_mutex_unlock_iothread();
         return 0;
 
     case IPMI_POWERCYCLE_CHASSIS: