hw/9pfs: Add st_gen support in getattr reply

hw/9pfs: Add st_gen support in getattr reply

This patch use file system specific ioctl for getting i_generation
value. Not all file system support the ioctl. So we add an export
specific extended operation and assign right callback for the
file system that support i_generation ioctl

["M. Mohan Kumar" <[email protected]> we can do ioctl only for
regular files and directories on the server]

Signed-off-by: Harsh Prateek Bora <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>