x86/paravirt: Restrict PARAVIRT_XXL to 64-bit only

PARAVIRT_XXL is exclusively utilized by XEN_PV, which is only compatible
with 64-bit machines.

Clearly designate PARAVIRT_XXL as 64-bit only and remove ifdefs to
support CONFIG_PGTABLE_LEVELS < 5.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250516123306.3812286-5-kirill.shutemov@linux.intel.com