VT-d/qinval: clean up error handling

VT-d/qinval: clean up error handling

- neither qinval_update_qtail() nor qinval_next_index() can fail: make
  the former return "void", and drop caller error checks for the latter
  (all of which would otherwise return with a spin lock still held)
- or-ing together error codes is a bad idea

At once drop bogus initializers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Yang Zhang <yang.z.zhang@intel.com>