HVM/save: allow the usage of zeroextend and a fixup function

HVM/save: allow the usage of zeroextend and a fixup function

With the current compat implementation in the save/restore context handling,
only one compat structure is allowed, and using _zeroextend prevents the
fixup function from being called.

In order to allow for the compat handling layer to be able to handle
different compat versions allow calling the fixup function with
hvm_load_entry_zeroextend.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>