tests/vhost-user-bridge.c: fix fd leakage

This fixes file descriptor leakage in vhost-user-bridge
application. Whenever a new callfd or kickfd is set, the previous
one should be explicitly closed. File descriptors used to map
guest's memory are closed immediately after mmap call.

Signed-off-by: Victor Kaplansky <victork@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
