qemu: fix regression with fd labeling on migration

qemu: fix regression with fd labeling on migration

My earlier testing for commit 34fa0de0 was done while starting
just-built libvirt from an unconfined_t shell, where the fds happened
to work when transferring to qemu.  But when installed and run under
virtd_t, failure to label the raw file (with no compression) or the
pipe (with compression) triggers SELinux failures when passing fds
over SCM_RIGHTS to svirt_t qemu.

* src/qemu/qemu_migration.c (qemuMigrationToFile): When passing
FDs, make sure they are labeled.