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>