conf: store mirroring information in virStorageSource

conf: store mirroring information in virStorageSource

The current implementation of 'virsh blockcopy' (virDomainBlockRebase)
is limited to copying to a local file name.  But future patches want
to extend it to also copy to network disks.  This patch converts over
to a virStorageSourcePtr, although it should have no semantic change
visible to the user, in anticipation of those future patches being
able to use more fields for non-file destinations.

* src/conf/domain_conf.h (_virDomainDiskDef): Change type of
mirror information.
* src/conf/domain_conf.c (virDomainDiskDefParseXML): Localize
mirror parsing into new object.
(virDomainDiskDefFormat): Adjust clients.
* src/qemu/qemu_domain.c (qemuDomainDeviceDefPostParse):
Likewise.
* src/qemu/qemu_driver.c (qemuDomainBlockPivot)
(qemuDomainBlockJobImpl, qemuDomainBlockCopy): Likewise.

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