tools/dombuilder: Drop more PVH v1 leftovers

tools/dombuilder: Drop more PVH v1 leftovers

alloc_magic_pages() is renamed to alloc_magic_pages_pv() to mirror its
alloc_magic_pages_hvm() counterpart.  Delete a redundant comment, introduce
some newlines clarity, and remove a logically dead allocation of shared info.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>