Actually use the enum in the domain conf structure.

* src/conf/domain_conf.h (_virDomainDiskDef): Store enum rather
than string for disk type.
* src/conf/domain_conf.c (virDomainDiskDefFree)
(virDomainDiskDefParseXML, virDomainDiskDefFormat)
(virDomainDiskDefForeachPath): Adjust users.
* src/xenxs/xen_sxpr.c (xenParseSxprDisks, xenFormatSxprDisk):
Likewise.
* src/xenxs/xen_xm.c (xenParseXM, xenFormatXMDisk): Likewise.
* src/vbox/vbox_tmpl.c (vboxAttachDrives): Likewise.
* src/libxl/libxl_conf.c (libxlMakeDisk): Likewise.