Consolidate printing of block driver options

Consolidate printing of block driver options

This consolidates the printing of block driver options in
print_block_option_help() which is called from both img_create() and
img_convert().

This allows for the "?" detection to be done just after the parsing of
options and the filename, instead of half way down the codepath of
these functions.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>