manpages/virsh: A couple of small clarifications

manpages/virsh: A couple of small clarifications

Changes:

  - Update the descriptions of --current & --config flags.

    For --config, the reason to rephrase "next boot" to "next start"
    is: "Next boot may still imply somebody selecting "reboot" in the
    guest OS and fully expecting the changes to be applied."  (per Peter
    Krempa)

    For --current, existing documentation says:

      "If *--current* is specified, affect the current guest state."

    It's not entirely clear what states can "current" mean or imply.  So
    rephrase it in context of the other two related flags --live and
    --config.

  - While at it, I also took the liberty to replace the few occurrences
    of "peristent domain[s]" with "persistent guest[s]"

Fix all occurrences (i.e. as many as I could spot) of this.

(Thanks: Dan Berrangé on IRC.)

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>