block/nfs: Introduce runtime_opts in NFS

Make NFS block driver use various fine grained runtime_opts.
Set .bdrv_parse_filename() to nfs_parse_filename() and introduce two
new functions nfs_parse_filename() and nfs_parse_uri() to help parsing
the URI.
Add a new option "server" which then accepts a new struct NFSServer.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
[ kwolf: Fixed client->path ]
Signed-off-by: Kevin Wolf <kwolf@redhat.com>