virtio: Decrement avail idx on buffer detach
virtio: Decrement avail idx on buffer detach

When detaching a buffer from a vq, the avail.idx value should be
decremented as well.

This was noticed by hot-unplugging a virtio console port and then
plugging in a new one on the same number (re-using the vqs which were
just 'disowned').  qemu reported

   'Guest moved used index from 0 to 256'

when any IO was attempted on the new port.

CC: 4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@kernel.org
Reported-by: juzhang <54a57e68d5fc42035cd89c02f8defc395d91d404@redhat.com>
Signed-off-by: Amit Shah <c7a04b2314b75cab042d4753bd03a193bc8ee50d@redhat.com>
Signed-off-by: Rusty Russell <df9728c9e5104131c08c7adb03af425394842596@rustcorp.com.au>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs