virttest.utils_misc: Fix functions that get qemu path

Since the repo split, we moved the original 'qemu',
'libvirt' and others to 'backends/qemu', 'backends/libvirt'
and similar. Let's fix the function that return qemu path
information.

Also, if the path informed in params is incorrect, try to
find whatever is installed system wide to use.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>