vl.c: Move option generation logic into a wrapper file

vl.c: Move option generation logic into a wrapper file

In vl.c and qemu-options.h we define macros and include qemu-options.def
in order to generate different content. Move the bulk of the def'ing and
undef'ing into a wrapper, this will make it cleaner when we add another
macro in the next patch.

AFAICS undefining GEN_DOCS services no purpose, but I've left it for now.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>