qga: update schema for guest-get-disks 'dependents' field

The recently-added 'guest-get-disk' command returns a list of
GuestDiskInfo entries, which in turn have a 'dependents' field which
lists devices these entries are dependent upon. Thus, 'dependencies'
is a better name for this field. Address this by renaming the field
accordingly.

Additionally, 'dependents' is specified as non-optional, even though
it's not implemented for w32. This is misleading, since it gives users
the impression that a particular disk might not have dependencies,
when in reality that information is simply not known to the guest
agent. Address this by making 'dependents' an optional field, and only
marking it as in-use when the facilities to obtain this information are
available to the guest agent.

Cc: Eric Blake <eblake@redhat.com>
Cc: Tomáš Golembiovský <tgolembi@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>