qemuDomainCreateDevice: Canonicalize paths

qemuDomainCreateDevice: Canonicalize paths

So far the decision whether /dev/* entry is created in the qemu
namespace is really simple: does the path starts with "/dev/"?
This can be easily fooled by providing path like the following
(for any considered device like disk, rng, chardev, ..):

  /dev/../var/lib/libvirt/images/disk.qcow2

Therefore, before making the decision the path should be
canonicalized.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>