virtiofs: define dax address space operations

This is done along the lines of ext4 and xfs.  I primarily wanted
->writepages hook at this time so that I could call into
dax_writeback_mapping_range().  This in turn will decide which pfns need to
be written back.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>