x86/images: convert oneof to enum + optional arg

Sadly, protobuf supports oneof keyword only from recent version 2.6.0.
On older environments protobuf-compiler will fail.
This is true for travis-ci with Ubuntu 14.04, where protobuf is 2.5.0.
Convert oneof to more stupid version with enum.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>

criu/arch/x86/crtools.c

@@ -198,7 +198,7 @@ int get_task_regs(pid_t pid, user_regs_struct_t regs, CoreEntry *core)
 		assign_reg(r64, regs.native, es);
 		assign_reg(r64, regs.native, fs);
 		assign_reg(r64, regs.native, gs);
-		gpregs->gpregs_case = USER_X86_REGS_ENTRY__GPREGS_NATIVE;
+		gpregs->gpregs_case = USER_X86_REGS_CASE_T__NATIVE;
 	} else {
 		UserX86Regs32 *r32 = gpregs->compat;
 
@@ -219,7 +219,7 @@ int get_task_regs(pid_t pid, user_regs_struct_t regs, CoreEntry *core)
 		assign_reg(r32, regs.compat, flags);
 		assign_reg(r32, regs.compat, sp);
 		assign_reg(r32, regs.compat, ss);
-		gpregs->gpregs_case = USER_X86_REGS_ENTRY__GPREGS_COMPAT;
+		gpregs->gpregs_case = USER_X86_REGS_CASE_T__COMPAT;
 	}
 
 #ifndef PTRACE_GETREGSET
@@ -558,7 +558,7 @@ void *mmap_seized(struct parasite_ctl *ctl,
 int restore_gpregs(struct rt_sigframe *f, UserX86RegsEntry *r)
 {
 	/* FIXME: rt_sigcontext for compatible tasks */
-	if (r->gpregs_case != USER_X86_REGS_ENTRY__GPREGS_NATIVE) {
+	if (r->gpregs_case != USER_X86_REGS_CASE_T__NATIVE) {
 		pr_err("Can't prepare rt_sigframe for compatible task restore\n");
 		return -1;
 	}

images/core-x86.proto

@@ -50,11 +50,15 @@ message user_x86_regs_32 {
 	required uint32			ss		= 17;
 }
 
+enum user_x86_regs_case_t {
+	NATIVE = 1;
+	COMPAT = 2;
+}
+
 message user_x86_regs_entry {
-	oneof gpregs {
-		user_x86_regs_64	native		= 1[(criu).hex = true];
-		user_x86_regs_32	compat		= 2[(criu).hex = true];
-	}
+	required user_x86_regs_case_t	gpregs_case	= 1;
+	optional user_x86_regs_64	native		= 2[(criu).hex = true];
+	optional user_x86_regs_32	compat		= 3[(criu).hex = true];
 }
 
 message user_x86_xsave_entry {