rutabaga: improve error handling, fix potential crash during init

When virtio_gpu_rutabaga_get_num_capsets() returns 0, virtio_init()
isn't called and the device later crashes during realize.
==72545==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x556ad6f7ba9e bp 0x7ffe6958f810 sp 0x7ffe6958f7e0 T0)
==72545==The signal is caused by a READ memory access.
==72545==Hint: address points to the zero page.
#0 0x556ad6f7ba9e in virtio_memory_listener_commit ../hw/virtio/virtio.c:4034
#1 0x556ad6a24c96 in listener_add_address_space ../system/memory.c:3128
#2 0x556ad6a25d15 in memory_listener_register ../system/memory.c:3216
#3 0x556ad6f7bf11 in virtio_device_realize ../hw/virtio/virtio.c:4075

Rework error handling of the function to set Error appropriately. 0
capset may be ok now.

Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>