xen/privcmd: make option visible in Kconfig

This configuration option provides a misc device as an API to userspace.
Make this API usable without having to select the module as a transitive
dependency.

This also fixes an issue where localyesconfig would select
CONFIG_XEN_PRIVCMD=m because it was not visible and defaulted to
building as module.

[boris: clarified help message per Jan's suggestion]
Based-on-patch-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211116143323.18866-1-jgross@suse.comReviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/Kconfig

@@ -259,9 +259,15 @@ config XEN_SCSI_BACKEND
 	  if guests need generic access to SCSI devices.
 
 config XEN_PRIVCMD
-	tristate
+	tristate "Xen hypercall passthrough driver"
 	depends on XEN
 	default m
+	help
+	  The hypercall passthrough driver allows privileged user programs to
+	  perform Xen hypercalls. This driver is normally required for systems
+	  running as Dom0 to perform privileged operations, but in some
+	  disaggregated Xen setups this driver might be needed for other
+	  domains, too.
 
 config XEN_ACPI_PROCESSOR
 	tristate "Xen ACPI processor"