vl: Avoid crash when -mon is underspecified

A quick coredump on an incomplete command line:
./x86_64-softmmu/qemu-system-x86_64 -mon mode=control,pretty=on

 #0  0x00007ffff723d9e4 in g_str_hash () at /lib64/libglib-2.0.so.0
 #1  0x00007ffff723ce38 in g_hash_table_lookup () at /lib64/libglib-2.0.so.0
 #2  0x0000555555cc0073 in object_class_property_find (klass=0x5555566a94b0, name=0x0, errp=0x0) at qom/object.c:1135
 #3  0x0000555555cc004b in object_class_property_find (klass=0x5555566a9440, name=0x0, errp=0x0) at qom/object.c:1129
 #4  0x0000555555cbfe6e in object_property_find (obj=0x5555568348c0, name=0x0, errp=0x0) at qom/object.c:1080
 #5  0x0000555555cc183d in object_resolve_path_component (parent=0x5555568348c0, part=0x0) at qom/object.c:1762
 #6  0x0000555555d82071 in qemu_chr_find (name=0x0) at chardev/char.c:802
 #7  0x00005555559d77cb in mon_init_func (opaque=0x0, opts=0x5555566b65a0, errp=0x0) at vl.c:2291

Fix it to instead fail gracefully.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20181023213600.364086-1-eblake@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NPeter Xu <peterx@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

vl.c

@@ -2323,6 +2323,10 @@ static int mon_init_func(void *opaque, QemuOpts *opts, Error **errp)
     }
 
     chardev = qemu_opt_get(opts, "chardev");
+    if (!chardev) {
+        error_report("chardev is required");
+        exit(1);
+    }
     chr = qemu_chr_find(chardev);
     if (chr == NULL) {
         error_setg(errp, "chardev \"%s\" not found", chardev);