conf, schema, docs: Add support for TSEG size setting

conf, schema, docs: Add support for TSEG size setting

TSEG (Top of Memory Segment) is one of many regions that SMM (System Management
Mode) can occupy.  This one, however is special, because a) most of the SMM code
lives in TSEG nowadays and b) QEMU just (well, some time ago) added support for
so called 'extended' TSEG.  The difference to the TSEG implemented in real q35's
MCH (Memory Controller Hub) is that it can offer one extra size to the guest OS
apart from the standard TSEG's 1, 2, and 8 MiB and that size can be selected in
1 MiB increments.  Maximum may vary based on QEMU and is way too big, so we
don't need to check for the maximum here.  Similarly to the memory size we'll
leave it to the hypervisor to try satisfying that and giving us an error message
in case it is not possible.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>