vhost: Fix typos

Fix multiple typos and improve comment clarity across vhost.c.

Spelling errors: "thead" -> "thread", "RUNNUNG" -> "RUNNING"
and "available".

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Message-Id: <20250615173933.1610324-1-alok.a.tiwari@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>