lvmstrap: add explicit test for swap backends

Similar to mounted filesystems, recent kernel/userland report swap
backends:

    root@node4:~# fuser -avm /dev/sda6
                         USER        PID ACCESS COMMAND
    /dev/sda6:           root     kernel swap  /dev/sda6

But old ones not:

    node1# fuser -avm /dev/sda6
                         USER        PID ACCESS COMMAND
    /dev/sda6:

So we add an explicit test for swap backends using /proc/swaps.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>