net: socket: do not close file descriptor if it's not a socket

JIRA: https://issues.redhat.com/browse/RHEL-582

The socket netdev with a file descriptor (fd) cannot be removed and then added again because the fd is closed when the backend is removed and thus is not available anymore when we want to add the backend again.

But this can bring to a core dump: 1- boot a VM with an fd socket netdev 2- remove the netdev 3- reboot 4- add the netdev again, it fails because the fd is not a socket, and then closed 5- stop QEMU -> core dump

On reboot (step 3) the fd is allocated to another use in QEMU, and when we try to use it with a socket netdev, it fails. But the netdev backend closes the file descriptor that is in use by another part of QEMU. We can see the core dump on QEMU exit because it tries to close an invalid file descriptor.

It happens for instance when we have a PCI device and the fd is allocated to a VirtIOIRQFD on reboot.

Moreover, using "netdev socket,fd=X" allows an user to close any QEMU internal file descriptor from an HMP or QMP interface.