docs: drvqemu: Fix and improve docs about device override types

docs: drvqemu: Fix and improve docs about device override types

The 'number' override type didn't exist in the final version so change
it to the corresponding 'signed' and 'unsigned'.

Additionally clarify which override type is used for a corresponding
qemu type and also that we use base 10 numbers so users will need to
convert the numbers if needed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>