iotests: Launch qemu-nbd with -e 42

There is no reason for the qemu-nbd server used for tests not to accept
an arbitrary number of clients. In fact, test 181 will require it to
accept two clients at the same time (and thus it fails before this
patch).

This patch updates common.rc to launch qemu-nbd with -e 42 which should
be enough for all of our current and future tests.

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