nvme: ensure the num_queues is not zero

nvme: ensure the num_queues is not zero

When it is zero, it causes segv.
Using following command:

"-drive file=//home/test/test1.img,if=none,id=id0
-device nvme,drive=id0,serial=test,num_queues=0"
causes following Backtrack:

Thread 4 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffe9735700 (LWP 30952)]
0x0000555555a7a77c in nvme_start_ctrl (n=0x5555577473f0) at hw/block/nvme.c:825
825	    if (unlikely(n->cq[0])) {
(gdb) bt
0  0x0000555555a7a77c in nvme_start_ctrl (n=0x5555577473f0)
    at hw/block/nvme.c:825
1  0x0000555555a7af7f in nvme_write_bar (n=0x5555577473f0, offset=20,
    data=4587521, size=4) at hw/block/nvme.c:969
2  0x0000555555a7b81a in nvme_mmio_write (opaque=0x5555577473f0, addr=20,
    data=4587521, size=4) at hw/block/nvme.c:1163
3  0x0000555555869236 in memory_region_write_accessor (mr=0x555557747cd0,
    addr=20, value=0x7fffe97320f8, size=4, shift=0, mask=4294967295, attrs=...)
    at /home/test/qemu1/qemu/memory.c:502
4  0x0000555555869446 in access_with_adjusted_size (addr=20,
    value=0x7fffe97320f8, size=4, access_size_min=2, access_size_max=8,
    access_fn=0x55555586914d <memory_region_write_accessor>,
    mr=0x555557747cd0, attrs=...) at /home/test/qemu1/qemu/memory.c:568
5  0x000055555586c479 in memory_region_dispatch_write (mr=0x555557747cd0,
    addr=20, data=4587521, size=4, attrs=...)
    at /home/test/qemu1/qemu/memory.c:1499
6  0x00005555558030af in flatview_write_continue (fv=0x7fffe0061130,
    addr=4273930260, attrs=..., buf=0x7ffff7ff0028 "\001", len=4, addr1=20,
    l=4, mr=0x555557747cd0) at /home/test/qemu1/qemu/exec.c:3234
7  0x00005555558031f9 in flatview_write (fv=0x7fffe0061130, addr=4273930260,
    attrs=..., buf=0x7ffff7ff0028 "\001", len=4)
    at /home/test/qemu1/qemu/exec.c:3273
8  0x00005555558034ff in address_space_write (
---Type <return> to continue, or q <return> to quit---
    as=0x555556758480 <address_space_memory>, addr=4273930260, attrs=...,
    buf=0x7ffff7ff0028 "\001", len=4) at /home/test/qemu1/qemu/exec.c:3363
9  0x0000555555803550 in address_space_rw (
    as=0x555556758480 <address_space_memory>, addr=4273930260, attrs=...,
    buf=0x7ffff7ff0028 "\001", len=4, is_write=true)
    at /home/test/qemu1/qemu/exec.c:3374
10 0x00005555558884a1 in kvm_cpu_exec (cpu=0x555556920e40)
    at /home/test/qemu1/qemu/accel/kvm/kvm-all.c:2031
11 0x000055555584cd9d in qemu_kvm_cpu_thread_fn (arg=0x555556920e40)
    at /home/test/qemu1/qemu/cpus.c:1281
12 0x0000555555dbaf6d in qemu_thread_start (args=0x5555569438a0)
    at util/qemu-thread-posix.c:502
13 0x00007ffff5dc86db in start_thread (arg=0x7fffe9735700)
    at pthread_create.c:463
14 0x00007ffff5af188f in clone ()
    at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190120055558.32984-3-liq3ea@163.com
Signed-off-by: Max Reitz <mreitz@redhat.com>