lguest: rename i386_head.S

Since commit 9a163ed8 (i386: move kernel) kernel/i386_head.S
was renamed to kernel/head_32.S. We do the same for lguest/i386_head.S.

Signed-off-by: Daniel Baluta <dbaluta@ixiacom.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>