iscsi: Handle -iscsi user/password in bdrv_parse_filename()

This splits the logic in the old parse_chap() function into a part that
parses the -iscsi options into the new driver-specific options, and
another part that actually applies those options (called apply_chap()
now).

Note that this means that username and password specified with -iscsi
only take effect when a URL is provided. This is intentional, -iscsi is
a legacy interface only supported for compatibility, new users should
use the proper driver-specific options.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
