qemu: snapshot: Improve detection of mixed snapshots

qemu: snapshot: Improve detection of mixed snapshots

Currently we don't support mixed (external + internal) snapshots. The
code detecting the snapshot type didn't make sure that the memory image
was consistent with the snapshot type leading into strange error
message:

 $ virsh snapshot-create-as --domain VM --diskspec vda,snapshot=internal --memspec snapshot=external,file=/tmp/blah
 error: internal error: unexpected code path

Fix the mixed detection code to detect this kind of mistake:

 $ virsh snapshot-create-as --domain VM --diskspec vda,snapshot=internal --memspec snapshot=external,file=/tmp/blah
 error: unsupported configuration: mixing internal and external targets for a snapshot is not yet supported