Fix some issue during create BootBackup

Using the follow parameters to enable iscsi boot:
BOOTIF=eth0 storage_init=/dev/vda,/dev/sda iscsi_init=/dev/sda iscsi_target_name=
iqn.2013-01.com:hadong iscsi_install iscsi_server=10.66.11.127:3260 firstboot.
The boot device /dev/vda wasn't managed by multipath
Auto install still failed with error info "Unable to find boot partition".
This is due to it create BootBackup using "mke2fs /dev/vdap3 -L BootBackup"
actually it should be "mke2fs /dev/vda3 -L BootBackup".

Change-Id: Ia0d202cc4a5c042f5be5527a151a1b0a55759e6a
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=885959
Signed-off-by: hadong <hadong0720@gmail.com>