use add_drive_ro for --mount parameters from guestfish when called with --ro

To prevent writes (such as ext3 journal replay) from occuring even when --ro is
passed, guestfish should use add_drive_ro() for any drives specified on the
command line with --add if --ro is also passed.

As we need to look through the entire command line for --ro before adding any
drives, we move the add process out of the argument-parsing loop and into its
own function, patterned off mount_mps().

Signed-off-by: Charles Duffy <charles_duffy@dell.com>