vhost: fix leak of fds and mmaps

vhost: fix leak of fds and mmaps

The common vhost code only supported a single mmap per device. vhost-user
worked around this by saving the address/length/fd of each mmap after the end
of the rte_virtio_memory struct. This only works if the vhost-user code frees
dev->mem, since the common code is unaware of the extra info. The
VHOST_USER_RESET_OWNER message is one situation where the common code frees
dev->mem and leaks the fds and mappings. This happens every time I shut down a
VM.

The new code calls back into the implementation (vhost-user or vhost-cuse) to
clean up these resources.

The vhost-cuse changes are only compile tested.

Signed-off-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>