qemu_domainjob: remove dependency on `qemuDomainDiskPrivatePtr`

qemu_domainjob: remove dependency on `qemuDomainDiskPrivatePtr`

Both parsing and formatting of NBD migration jobs is QEMU specific and
since we're trying to create a hypervisor-agnostic module out of
qemu_domainjob.c, move the NBD XML handling bits to the qemu_domain
module instead. Additionally, move the respective NBD XML calls to
the 'parseJob'/'formatJob' callbacks of the
qemuDomainObjPrivateJobCallbacks structure.

Signed-off-by: Prathamesh Chavan <pc44800@gmail.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>