qemu: use -incoming fd:n to avoid qemu holding fd indefinitely

qemu: use -incoming fd:n to avoid qemu holding fd indefinitely

https://bugzilla.redhat.com/show_bug.cgi?id=620363

When using -incoming stdio or -incoming exec:, qemu keeps the
stdin fd open long after the migration is complete.  Not to
mention that exec:cat is horribly inefficient, by doubling the
I/O and going through a popen interface in qemu.

The new -incoming fd: of qemu 0.12.0 closes the fd after using
it, and allows us to bypass an intermediary cat process for
less I/O.

* src/qemu/qemu_command.h (qemuBuildCommandLine): Add parameter.
* src/qemu/qemu_command.c (qemuBuildCommandLine): Support
migration via fd: when possible.  Consolidate migration handling
into one spot, now that it is more complex.
* src/qemu/qemu_driver.c (qemudStartVMDaemon): Update caller.
* tests/qemuxml2argvtest.c (mymain): Likewise.
* tests/qemuxml2argvdata/qemuxml2argv-restore-v2-fd.args: New file.
* tests/qemuxml2argvdata/qemuxml2argv-restore-v2-fd.xml: Likewise.