vbox: Rewrite vboxDomainSnapshotCreateXML

vbox: Rewrite vboxDomainSnapshotCreateXML

The vboxDomainSnapshotCreateXML integrated the snapshot redefine
with this patch:
http://www.redhat.com/archives/libvir-list/2014-May/msg00589.html
This patch introduced vboxSnapshotRedefine in vboxUniformedAPI to
enable the features.

This patch replace all version specified APIs to the uniformed api,
then, moving the whole implementation to vbox_common.c. As there
is only API level changes, the behavior of the function doesn't
change.

Some old version's defects has brought to the new one. The already
known things are:
    *goto cleanup in a loop without releasing the pointers in the
    loop.
    *When function failed after machine unregister, no roll back
    to recovery it and the virtual machine would disappear.