[BLKTAP][XEND] Fix blktap to work with a bootloader

[BLKTAP][XEND] Fix blktap to work with a bootloader
If a Xen guest has a bootloader configured, then it will fail to start
on a blktap image.  The problem is blkdev_uname_to_file, which cannot
parse the "tap:aio:$filename" image strings: it tries to split the
string apart at ":" and assign the result to a 2-tuple, and this
results in a python error if the split results in three or more
strings.

The fix is to split only at the first ":", and then to split again
if we detect "tap:" as the image type.

Signed-off-by: Stephen Tweedie <sct@redhat.com>