os_dep: add '/usr/libexec' in dir list to find command in os_dep.command() fucntion.

Some command in /usr/libexec is also we need, such as qemu-kvm and libvirt_lxc,
but this dir is not in default PATH in RHEL. Then this patch find command in
/usr/libexec in os_dep.command() function.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>