discover-image: introduce bus_property_get_image_is_read_only() with BUS_DEFINE_PROPERTY_GET() macro

This also makes image_is_read_only() return bool.

Follow-up for ee327e086e.

src/shared/discover-image.c

@@ -16,6 +16,7 @@
 #include "alloc-util.h"
 #include "blockdev-util.h"
 #include "btrfs-util.h"
+#include "bus-get-properties.h"
 #include "chase.h"
 #include "chattr-util.h"
 #include "copy.h"
@@ -2220,3 +2221,5 @@ int image_root_pick(
         *ret = TAKE_PTR(s);
         return 0;
 }
+
+BUS_DEFINE_PROPERTY_GET(bus_property_get_image_is_read_only, "b", Image, (int) image_is_read_only);

src/shared/discover-image.h

@@ -91,7 +91,7 @@ static inline bool image_is_hidden(const Image *i) {
         return i->name && i->name[0] == '.';
 }
 
-static inline int image_is_read_only(const Image *i) {
+static inline bool image_is_read_only(const Image *i) {
         assert(i);
 
         /* We enforce the rule that hidden images are always read-only too. If people want to change hidden
@@ -102,6 +102,7 @@ static inline int image_is_read_only(const Image *i) {
 
         return i->read_only;
 }
+int bus_property_get_image_is_read_only(sd_bus *bus, const char *path, const char *interface, const char *property, sd_bus_message *reply, void *userdata, sd_bus_error *reterr_error);
 
 bool image_is_vendor(const Image *i);
 bool image_is_host(const Image *i);