Documentation: admin-guide: Update kvm/xen config option

Since commit 9bba03d4

 ("kconfig: remove 'kvmconfig' and 'xenconfig'
shorthands") kvm/xen config shortcuts are not available anymore. Update
the file to reflect how they should be used, with the full filename.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Link: https://lore.kernel.org/r/20210130014547.123006-2-andrealmeid@collabora.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>