docs: Fix information for default TPM version
docs: Fix information for default TPM version

The current information is not accurate, because the default
is 2.0 instead of 1.2 for the tpm-crb and tpm-spapr models.

Any detailed list will surely become obsolete and out of sync
with reality over time, so let's just document that the default
model depends on a number of factors and avoid getting any more
specific than that.

Signed-off-by: Andrea Bolognani <3ada0bee826c753786fdbba72243dfba997094cf@redhat.com>
Tested-by: Liu Yiding <3d7f5c4cbb1b004d4b3bae6b38444f1a767136d5@fujitsu.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,olafhering/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,libvirt/libvirt,crobinso/libvirt,nertpinx/libvirt