Use virURIFree instead of xmlFreeURI

Use virURIFree instead of xmlFreeURI

Since we defined a custom virURIPtr type, we should use a
virURIFree method instead of assuming it will always be
a typedef for xmlURIPtr

* src/util/viruri.c, src/util/viruri.h, src/libvirt_private.syms:
  Add a virURIFree method
* src/datatypes.c, src/esx/esx_driver.c, src/libvirt.c,
  src/qemu/qemu_migration.c, src/vmx/vmx.c, src/xen/xend_internal.c,
  tests/viruritest.c: s/xmlFreeURI/virURIFree/

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>