qemu: monitor: Extract vmstate presence for internal snapshots in qemuBlockGetNamedNo...

qemu: monitor: Extract vmstate presence for internal snapshots in qemuBlockGetNamedNodeData

Refactor the parts of qemuBlockGetNamedNodeData which fetch the names of
internal snapshots present in the on-disk state of QCOW2 images to also
extract the presence of the 'vmstate' section.

This requires conversion of the snapshot list to a hash table as we
always know the name of the snapshot that we're looking for, and the
hash table allows also storing of additional data which we'll use to
store the presence of the 'vmstate'.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>