scsi-generic: check the return value of bdrv_aio_ioctl in execute_command

This fixes the bug introduced by this commit ad54ae80c73f.
The bdrv_aio_ioctl() still could return null and we should return an error
in that case.

Cc: qemu-stable@nongnu.org
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
