add-domain: Add readonlydisk optional argument (RHBZ#747290).

add-domain: Add readonlydisk optional argument (RHBZ#747290).

This optional argument controls how <readonly/> disks in
the libvirt XML are handled.  It can be set to one of:

  "write" (default) - add them R/W
  "read" - add them R/O
  "error" - throw an error if asked to add them R/W
  "ignore" - ignore these disks

I have added limited regression tests for this feature.  However
libvirt's test:/// XML does not allow you to specify that a domain
starts off shut down, so we cannot fully test this.  Instead I tested
it by hand.