qemu_shim: URI escape root directory

qemu_shim: URI escape root directory

The root directory can be provided by user (or a temporary one is
generated) and is always formatted into connection URI for both
secret driver and QEMU driver, like this:

  qemu:///embed?root=$root

But if it so happens that there is an URI unfriendly character in
root directory or path to it (say a space) then invalid URI is
formatted which results in unexpected results. We can trust
g_dir_make_tmp() to generate valid URI but we can't trust user.
Escape user provided root directory. Always.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1920400
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>