qemu-io: Make alloc output useful when nb_sectors=1

qemu-io: Make alloc output useful when nb_sectors=1

There is no indication whether or not the sector is allocated when
nb_sectors=1:

  sector allocated at offset 64 KiB

This message is produced whether or not the sector is allocated.

Simply use the same message as the plural case, I don't think the
English is so broken that we need special case output here:

  0/1 sectors allocated at offset 64 KiB

This change does not affect qemu-iotests since nb_sectors=1 is not used
there.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>