Without setting the id, calling xstats_get twice with same array causes
memory corruption.
Also, if IXGBEVF_NB_XSTATS will be different than 1 in the future,
this will cause issues.
Fixes: 156712ba40d3 ("ixgbevf: add extending stats")
Signed-off-by: Ido Barnea <ibarnea@cisco.com>