fix #4101: acquire job's aio context before calling job_unref

fix #4101: acquire job's aio context before calling job_unref

Otherwise, we might run into an abort via bdrv_co_yield_to_drain()
(can at least happen when a disk with iothread is used):
> #0  0x00007fef4f5dece1 __GI_raise (libc.so.6 + 0x3bce1)
> #1  0x00007fef4f5c8537 __GI_abort (libc.so.6 + 0x25537)
> #2  0x00005641bce3c71f error_exit (qemu-system-x86_64 + 0x80371f)
> #3  0x00005641bce3d02b qemu_mutex_unlock_impl (qemu-system-x86_64 + 0x80402b)
> #4  0x00005641bcd51655 bdrv_co_yield_to_drain (qemu-system-x86_64 + 0x718655)
> #5  0x00005641bcd52de8 bdrv_do_drained_begin (qemu-system-x86_64 + 0x719de8)
> #6  0x00005641bcd47e07 blk_drain (qemu-system-x86_64 + 0x70ee07)
> #7  0x00005641bcd498cd blk_unref (qemu-system-x86_64 + 0x7108cd)
> #8  0x00005641bcd31e6f block_job_free (qemu-system-x86_64 + 0x6f8e6f)
> #9  0x00005641bcd32d65 job_unref (qemu-system-x86_64 + 0x6f9d65)
> #10 0x00005641bcd93b3d pvebackup_co_complete_stream (qemu-system-x86_64 + 0x75ab3d)
> #11 0x00005641bce4e353 coroutine_trampoline (qemu-system-x86_64 + 0x815353)

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
Acked-by: Wolfgang Bumiller <w.bumiller@proxmox.com>