testutilsqemu: Helpers for changing host CPU and arch
    
    Changing a host architecture or a CPU is not as easy as assigning a new
    value to the appropriate element in virCaps since there is a relation
    between the CPU and host architecture (we don't really want to test
    anything on an AArch64 host with core2duo CPU). This patch introduces
    qemuTestSetHostArch and qemuTestSetHostCPU helpers which will make sure
    the host architecture matches the host CPU.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>