qemuProcessStart: Rename 'snapshot' to 'internalSnapshotRevert'

qemuProcessStart: Rename 'snapshot' to 'internalSnapshotRevert'

Make it obvious that the variable is used for internal snapshot
reversion by renaming it. This is necessary mainly as the function
parameters are not documented, but makes it obvious also if they were.

We can also report the name of the sanpshot rather than a pointer that
says absolutely nothing to the reader.

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