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: John Ferlan <jferlan@redhat.com>