iotests: Fix 083 for out-of-tree builds

iotest 083 filters out debug messages from nbd, which are prefixed (and
recognized) by __FILE__. However, the current filter (/^nbd\.c…/) is
valid for in-tree builds only, as out-of-tree builds will have a path
before that filename (e.g. "/tmp/qemu/nbd.c"). Fix this by adding .*
before "nbd\.c".

While working on this, also fix the regexes: '.' should be escaped and a
single backslash is not enough for escaping when enclosed by double
quotes.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
