With this patch, all information related to a host resource in
a storage file backing chain now lives in util/virstoragefile.h.
The next step will be to consolidate various places that have
been tracking backing chain details to all use a common struct.

The changes to tools/Makefile.am were made necessary by the
fact that virstorageencryption includes uses of libxml, and is
now pulled in by inclusion from virstoragefile.h.  No
additional libraries are linked into the final image, and in
comparison, the build of the setuid library in src/Makefile.am
already was using LIBXML_CFLAGS via AM_CFLAGS.

* src/conf/domain_conf.h (virDomainDiskSourceDef): Move...
* src/util/virstoragefile.h (virStorageSource): ...and rename.
* src/conf/domain_conf.c (virDomainDiskSourceDefClear)
(virDomainDiskAuthClear): Adjust clients.
* tools/Makefile.am (virt_login_shell_CFLAGS)
(virt_host_validate_CFLAGS): Add libxml headers.
Signed-off-by: NEric Blake <eblake@redhat.com>