x86/HVM: move per-vendor function tables into .init.data

x86/HVM: move per-vendor function tables into .init.data

hvm_enable() copies the table contents rather than storing the pointer,
so there's no need to keep these tables post-boot.

Also constify the return values of the per-vendor initialization
functions, making clear that once the per-vendor initialization is
complete, the vendor specific tables won't get modified anymore.

Finally, in hvm_enable(), use the returned pointer for all read
accesses as being more efficient than global variable accesses. Writes
of course still need to go to the global variable.

Signed-off-by: Jan Beulich <jbeulich@suse.com>