The blkback name is like blkback.domid.xvd[a-z], if domid has four digits
(means larger than 1000), then the backmost xvd wouldn't be fully shown.

Define a BLKBACK_NAME_LEN macro to be 20, enlarge the array size of
blkback name, so it will be fully shown in any case.
Signed-off-by: NTao Chen <boby.chen@huawei.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: NRoger Pau Monné <roger.pau@citrix.com>