hw/9pfs: make get_st_gen() return ENOTTY error on special files

hw/9pfs: make get_st_gen() return ENOTTY error on special files

Currently we silently ignore getversion requests for anything except
file or directory. Let's instead return ENOTTY error to indicate that
getversion is not supported. It makes implementation consistent on
all not-supported cases.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>