run: fallback to qemu-img in /usr/bin

In case there is no qemu-img next to the qemu binary try /usr/bin/qemu-img (and
qemu-io) as fallback.  Makes "run --qemu-bin=/usr/libexec/qemu-kvm" work.  Likewise
"run --qemu-bin=/home/kraxel/rhel6/qemu-kvm/x86_64-softmmu/qemu-system-x86_64".
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>