Using the query-qmp-schema introspection - look for the 'vxhs'
blockdevOptions type.

NB: This is a "best effort" type situation as there is not a
    mechanism to determine whether the running QEMU has been
    built with '--enable-vxhs'. All we can do is check if the
    option to use vxhs for a blockdev-add exists in the command
    infrastructure which does not take that into account when
    building its table of commands and options.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>