conf: accessors for common source information

conf: accessors for common source information

A future patch will split virDomainDiskDef, in order to track
multiple host resources per guest <disk>.  To reduce the size
of that patch, I've factored out the four most common accesses
into functions, so that I can incrementally upgrade the code
base to use the accessors, and so that code that doesn't care
about the distinction of per-file details won't have to be
changed when the struct changes.

* src/conf/domain_conf.h (virDomainDiskGetType)
(virDomainDiskSetType, virDomainDiskGetSource)
(virDomainDiskSetSource, virDomainDiskGetDriver)
(virDomainDiskSetDriver, virDomainDiskGetFormat)
(virDomainDiskSetFormat): New prototypes.
* src/conf/domain_conf.c (virDomainDiskGetType)
(virDomainDiskSetType, virDomainDiskGetSource)
(virDomainDiskSetSource, virDomainDiskGetDriver)
(virDomainDiskSetDriver, virDomainDiskGetFormat)
(virDomainDiskSetFormat): Implement them.
* src/libvirt_private.syms (domain_conf.h): Export them.

Signed-off-by: Eric Blake <eblake@redhat.com>