virtio: use virtio_device_ready() in virtio_device_restore()

In the Linux kernel, the following vulnerability has been resolved:

virtio: use virtio_device_ready() in virtio_device_restore()

After waking up a suspended VM, the kernel prints the following trace

for virtio drivers which do not directly call virtio_device_ready() in

the .restore:

PM: suspend exit

irq 22: nobody cared (try booting with the "irqpoll" option)

Call Trace:

<IRQ>

dump_stack_lvl+0x38/0x49

dump_stack+0x10/0x12

__report_bad_irq+0x3a/0xaf

note_interrupt.cold+0xb/0x60

handle_irq_event+0x71/0x80

handle_fasteoi_irq+0x95/0x1e0

__common_interrupt+0x6b/0x110

common_interrupt+0x63/0xe0

asm_common_interrupt+0x1e/0x40

? __do_softirq+0x75/0x2f3

irq_exit_rcu+0x93/0xe0

sysvec_apic_timer_interrupt+0xac/0xd0

</IRQ>

<TASK>

asm_sysvec_apic_timer_interrupt+0x12/0x20

arch_cpu_idle+0x12/0x20

default_idle_call+0x39/0xf0

do_idle+0x1b5/0x210

cpu_startup_entry+0x20/0x30

start_secondary+0xf3/0x100

secondary_startup_64_no_verify+0xc3/0xcb

</TASK>

handlers:

[<000000008f9bac49>] vp_interrupt

[<000000008f9bac49>] vp_interrupt

Disabling IRQ #22

This happens because we don't invoke .enable_cbs callback in

virtio_device_restore(). That callback is used by some transports

(e.g. virtio-pci) to enable interrupts.

Let's fix it, by calling virtio_device_ready() as we do in

virtio_dev_probe(). This function calls .enable_cts callback and sets

DRIVER_OK status bit.

This fix also avoids setting DRIVER_OK twice for those drivers that

call virtio_device_ready() in the .restore.