qemu: Fix crash in qemucapsprobe

qemu: Fix crash in qemucapsprobe

The qemucapsprobe helper calls virQEMUCapsNewForBinaryInternal with
caps == NULL, causing the following crash:

    Program received signal SIGSEGV, Segmentation fault.
    #0  0x00007ffff788775f in virQEMUCapsInitHostCPUModel
        (qemuCaps=qemuCaps@entry=0x649680, host=host@entry=0x10) at
        src/qemu/qemu_capabilities.c:2969
    #1  0x00007ffff7889dbf in virQEMUCapsNewForBinaryInternal
        (caps=caps@entry=0x0, binary=<optimized out>,
        libDir=libDir@entry=0x4033f6 "/tmp", cacheDir=cacheDir@entry=0x0,
        runUid=runUid@entry=4294967295, runGid=runGid@entry=4294967295,
        qmpOnly=true) at src/qemu/qemu_capabilities.c:4039
    #2  0x0000000000401702 in main (argc=2, argv=0x7fffffffd968) at
        tests/qemucapsprobe.c:73

Caused by v2.2.0-182-g68c7011.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>