vtnet: expose features via sysctl tree

Right now the 64-bit flags field needs to be casted to a 32-bit field,
because clang warns if more than 32-bits are used.
Once clang is fixed, this restriction will be removed and more bits
will be added.

Reviewed by:		markj, Timo Völker
MFC after:		1 week
Differential Revision:	https://reviews.freebsd.org/D54288

share/man/man4/vtnet.4

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd December 18, 2025
+.Dd December 19, 2025
 .Dt VTNET 4
 .Os
 .Sh NAME
@@ -274,6 +274,8 @@ The maximum number of supported virtqueue pairs.
 .It Va dev.vtnet. Ns Ar X Ns Va .flags
 The flags of the interface.
 Mostly for debugging purposes.
+.It Va dev.vtnet. Ns Ar X Ns Va .features
+The features of the interface as defined by the virtio specification.
 .El
 .Sh SEE ALSO
 .Xr arp 4 ,