i386 max_cpus off by one
Description of problem:
X86 VMs are currently limited to 255 vCPUs (`mc->max_cpus = 255;` in `pc.c`).
The first occurrence i can find of this limit is in d3e9db933f416c9f1c04df4834d36e2315952e42 from 2005 where both `MAX_APICS` and `MAX_CPUS` was set to 255. This is becoming relevant for some people as servers with 256 cores become more available. 

**Can we increase the limit to 256 vCPUs?** 
I think so. 

Today, the APIC id limit (see `apic_id_limit` in `x86-common.c`) is based on the CPU id limit. 
According to the a comment for `typdef uint32_t apic_id_t;` (see `topology.h`), we can have 256 APICs, but more APICs require x2APIC support. 
APIC seems to be no hindrance to increase max_cpus to 256. 

**Can we increase the limit to 512?** Maybe not? We need x2APIC support of which i have no clue. Also there is always a performance risk of exceeding the size at which current data structures work efficiently.