Calling the monitor was convenient for the implementation in
qemuDomainBlockCopyCommon, but causes the snapshot code to call
query-named-block-nodes for every disk.

Fix this by removing the monitor call from
qemuBlockStorageSourceCreateDetectSize so that the data can be reused in
loops.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
ACKed-by: NEric Blake <eblake@redhat.com>