Introduce rerror option for drives

rerror controls the action to be taken when an error occurs while accessing the
guest image file. It corresponds to werror which already controls the action
take for write errors.

This purely introduces parsing rerror command line option into the right
structures, real support for it in the device emulation is added in the
following patches.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>