Ensure 'arch' is always set in cpuArchNodeData

Ensure 'arch' is always set in cpuArchNodeData

The s390, ppc and arm CPU drivers never set the 'arch' field
in their impl of cpuArchNodeData. This leads to error messages
being reported from cpuDataFree later, due to trying to use
VIR_ARCH_NONE.

 #0  virRaiseErrorFull (filename=filename@entry=0x76f94434 "cpu/cpu.c", funcname=funcname@entry=0x76f942dc <__FUNCTION__.18096> "cpuGetSubDriver", linenr=linenr@entry=58,
     domain=domain@entry=31, code=code@entry=1, level=level@entry=VIR_ERR_ERROR, str1=0x76f70e18 "internal error: %s",
     str2=str2@entry=0x7155f2ec "undefined hardware architecture", str3=str3@entry=0x0, int1=int1@entry=-1, int2=int2@entry=-1, fmt=0x76f70e18 "internal error: %s")
     at util/virerror.c:646
 #1  0x76e682ea in virReportErrorHelper (domcode=domcode@entry=31, errorcode=errorcode@entry=1, filename=0x76f94434 "cpu/cpu.c",
     funcname=0x76f942dc <__FUNCTION__.18096> "cpuGetSubDriver", linenr=linenr@entry=58, fmt=0x76f7e7e4 "%s") at util/virerror.c:1292
 #2  0x76ed82d4 in cpuGetSubDriver (arch=<optimized out>) at cpu/cpu.c:57
 #3  cpuGetSubDriver (arch=VIR_ARCH_NONE) at cpu/cpu.c:51
 #4  0x76ed8818 in cpuDataFree (data=data@entry=0x70c22d78) at cpu/cpu.c:216
 #5  0x716aaec0 in virQEMUCapsInitCPU (arch=VIR_ARCH_ARMV7L, caps=0x70c29a08) at qemu/qemu_capabilities.c:867

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>