snapshot: make it possible to audit external snapshot

snapshot: make it possible to audit external snapshot

Snapshots alter the set of disk image files opened by qemu, so
they must be audited.  But they don't involve a full disk definition
structure, just the new filename.  Make the next patch easier by
refactoring the audit routines to just operate on file name.

* src/conf/domain_audit.h (virDomainAuditDisk): Update prototype.
* src/conf/domain_audit.c (virDomainAuditDisk): Act on strings,
not definition structures.
(virDomainAuditStart): Update caller.
* src/qemu/qemu_hotplug.c (qemuDomainChangeEjectableMedia)
(qemuDomainAttachPciDiskDevice, qemuDomainAttachSCSIDisk)
(qemuDomainAttachUsbMassstorageDevice)
(qemuDomainDetachPciDiskDevice, qemuDomainDetachDiskDevice):
Likewise.