docs/vtpm: fix auto-shutdown reference

docs/vtpm: fix auto-shutdown reference

The automatic shutdown feature of the vTPM was removed because it
interfered with pv-grub measurement support and was also not triggered
if the guest did not use the vTPM. Virtual TPM domains will need to be
shut down or destroyed on guest shutdown via a script or other user
action.

This also fixes an incorrect reference to the vTPM being PV-only.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>