docs: correct improper information about domain states in virsh manpage

docs: correct improper information about domain states in virsh manpage

Commit 24d4a0a1f removed the non-existent "dying" state from the list
of possible domain states given in the virsh manpage, but didn't
correct the count of states from 8 down to 7. This patch fixes that
mismatch by completely removing any reference to the exact number of
states (thus preventing a potential future mismatch), while wording
the sentence in a more readable/truthful manner.