vhost: fix leak of file descriptors

vhost: fix leak of file descriptors

While migration of vhost-user device QEMU allocates memfd
to store information about dirty pages and sends fd to
vhost-user process.

File descriptor for this memory should be closed to prevent
"Too many open files" error for vhost-user process after
some amount of migrations.

Ex.:
 # ls /proc/<ovs-vswitchd pid>/fd/ -alh
 total 0
 root qemu  .
 root qemu  ..
 root qemu  0 -> /dev/pts/0
 root qemu  1 -> pipe:[1804353]
 root qemu  10 -> socket:[1782240]
 root qemu  100 -> /memfd:vhost-log (deleted)
 root qemu  1000 -> /memfd:vhost-log (deleted)
 root qemu  1001 -> /memfd:vhost-log (deleted)
 root qemu  1004 -> /memfd:vhost-log (deleted)
 [...]
 root qemu  996 -> /memfd:vhost-log (deleted)
 root qemu  997 -> /memfd:vhost-log (deleted)

 ovs-vswitchd.log:
 |WARN|punix:ovs-vswitchd.ctl: accept failed: Too many open files

Fixes: 54f9e32305d4 ("vhost: handle dirty pages logging request")

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>