i386: Define static "base" CPU model

i386: Define static "base" CPU model

The query-cpu-model-expand QMP command needs at least one static
model, to allow the "static" expansion mode to be implemented.
Instead of defining static versions of every CPU model, define a
"base" CPU model that has absolutely no feature flag enabled.

Despite having no CPUID data set at all, "-cpu base" is even a
functional CPU:

* It can boot a Slackware Linux 1.01 image with a Linux 0.99.12
  kernel[1].
* It is even possible to boot[2] a modern Fedora x86_64 guest by
  manually enabling the following CPU features:
  -cpu base,+lm,+msr,+pae,+fpu,+cx8,+cmov,+sse,+sse2,+fxsr

[1] http://www.qemu-advent-calendar.org/2014/#day-1
[2] This is what can be seen in the guest:
    [root@localhost ~]# cat /proc/cpuinfo
    processor       : 0
    vendor_id       : unknown
    cpu family      : 0
    model           : 0
    model name      : 00/00
    stepping        : 0
    physical id     : 0
    siblings        : 1
    core id         : 0
    cpu cores       : 1
    apicid          : 0
    initial apicid  : 0
    fpu             : yes
    fpu_exception   : yes
    cpuid level     : 1
    wp              : yes
    flags           : fpu msr pae cx8 cmov fxsr sse sse2 lm nopl
    bugs            :
    bogomips        : 5832.70
    clflush size    : 64
    cache_alignment : 64
    address sizes   : 36 bits physical, 48 bits virtual
    power management:

    [root@localhost ~]# x86info -v -a
    x86info v1.30.  Dave Jones 2001-2011
    Feedback to <davej@redhat.com>.

    No TSC, MHz calculation cannot be performed.
    Unknown vendor (0)
    MP Table:

    Family: 0 Model: 0 Stepping: 0
    CPU Model (x86info's best guess):

    eax in: 0x00000000, eax = 00000001 ebx = 00000000 ecx = 00000000 edx = 00000000
    eax in: 0x00000001, eax = 00000000 ebx = 00000800 ecx = 00000000 edx = 07008161

    eax in: 0x80000000, eax = 80000001 ebx = 00000000 ecx = 00000000 edx = 00000000
    eax in: 0x80000001, eax = 00000000 ebx = 00000000 ecx = 00000000 edx = 20000000

    Feature flags:
     fpu            Onboard FPU
     msr            Model-Specific Registers
     pae            Physical Address Extensions
     cx8            CMPXCHG8 instruction
     cmov           CMOV instruction
     fxsr           FXSAVE and FXRSTOR instructions
     sse            SSE support
     sse2           SSE2 support

    Long NOPs supported: yes

    Address sizes : 0 bits physical, 0 bits virtual
    0MHz processor (estimate).

     running at an estimated 0MHz
    [root@localhost ~]#

Message-Id: <20170222190029.17243-2-ehabkost@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>