Update ioctl order in nbd_init() to detect EBUSY

Update ioctl order in nbd_init() to detect EBUSY

Update ioctl(s) in nbd_init() to detect device busy early.

Current nbd_init() issues NBD_CLEAR_SOCKET before NBD_SET_SOCKET, if issuing
"qemu-nbd -c /dev/nbd0 disk.img" twice, the second time won't detect EBUSY in
nbd_init(), but in nbd_client will report EBUSY and do clear socket (the 1st
time command will be affacted too because of no socket any more.)

No change to previous version.

Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>