qemu: consolidate migration to file code

qemu: consolidate migration to file code

This points out that core dumps (still) don't work for root-squash
NFS, since the fd is not opened correctly.  This patch should not
introduce any functionality change, it is just a refactoring to
avoid duplicated code.

* src/qemu/qemu_migration.h (qemuMigrationToFile): New prototype.
* src/qemu/qemu_migration.c (qemuMigrationToFile): New function.
* src/qemu/qemu_driver.c (qemudDomainSaveFlag, doCoreDump): Use
it.