vm/qrexec: fix race between child cleanup and select call
vm/qrexec: fix race between child cleanup and select call

reap_children() can close FD, which was already added to FD_SET for select.
This can lead to EBADF and agent termination.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin