Revert "add_drive_ro adds readonly=on option if available." (RHBZ#617200).

Revert "add_drive_ro adds readonly=on option if available." (RHBZ#617200).

Adding the readonly=on option is not so clever.  This causes
qemu to present the disk as read-only to the guest.  (The
expected behaviour of snapshots=on,readonly=on was that it
would open the disk O_RDONLY but present a writable disk to
the guest).

Since the guest sees a read-only disk, we are unable to do any
recovery if a filesystem on the disk is inconsistent.  This basically
prevents most accesses to live disk images.

What we really want is a qemu option which presents a writable
disk to the guest, but only opens the disk on the host side with
O_RDONLY, to alleviate the udev bug RHBZ#571714.

This reverts commit 676462684e05dd8341dd695762dd99a87d8ec022.