x86/paravirt: Move some functions and defines to alternative.c

As a preparation for replacing paravirt patching completely by
alternative patching, move some backend functions and #defines to
the alternatives code and header.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231129133332.31043-3-jgross@suse.com