Correct autodetection of bootfs property

Remove lines that contain only a hyphen (match '^-$' instead of '-').

I had a root fs with a hyphen in the name (fedora/ROOT/Fedora20-Dev),
it was not detected because sed eliminated that line of output from
'zpool list -Ho bootfs'.

Signed-off-by: Evan Susarret <evansus@gmail.com>
Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2196