utils_misc: Return default qemu if specified qemu binary not available.

Do not have qemu/libvirt sub folder under virt-test repo root folder.

So get_qemu_binary functions will return a wrong qemu link.
Update get_qemu_binary to return default qemu in host if specified
qemu binary not available.

Also try to find qemu link under root folder (data_dir.get_root_dir())
Do we have more reasonable folder for these files?

Signed-off-by: Feng Yang <fyang@redhat.com>