drives: Use libxml2 to construct safely quoted URIs for qemu.
drives: ceph/gluster/iscsi/sheepdog/ssh: Force the disk name to begin with a '/'.

This avoids confusion when using URIs in guestfish, since the path
will always start with a '/', and we don't otherwise know if we should
remove it or not.  By forcing the '/' to always be there, we deal with
this problem in the API instead.
drives: Remove check for ':' in filename (RHBZ#811649).

Modern qemu can now handle this properly.  ':' is only special if what
precedes it looks like a transport, so:

  qemu-system-x86_64 -drive foo:bar     .. fails
  qemu-system-x86_64 -drive ./foo:bar   .. works

Thus by adding ./ in front of relative paths that contain ':' we can
work around this.

In addition, this broke iscsi:// URIs because iSCSI target names
routinely contain ':' characters.