Add some fail points to gmirror.

These are useful for testing changes to I/O error handling, and for
reproducing existing bugs in a controlled manner. The fail points are

g_mirror_regular_request_read
g_mirror_regular_request_write
g_mirror_sync_request_read
g_mirror_sync_request_write
g_mirror_metadata_write

They all effectively allow one to inject an error value into the bio_error
field of a corresponding BIO request as it is being completed.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division