This is trivial for QEMU since you just have to not stop the vm before
starting the dump.  And for Xen, you just pass the flag down to xend.

* include/libvirt/libvirt.h.in (virDomainCoreDumpFlags): Add VIR_DUMP_LIVE.
* src/qemu/qemu_driver.c (qemudDomainCoreDump): Support live dumping.
* src/xen/xend_internal.c (xenDaemonDomainCoreDump): Support live dumping.
* tools/virsh.c (opts_dump): Add --live. (cmdDump): Map it to VIR_DUMP_LIVE.