Change ZEVENT_POOL_GUID to ZEVENT_POOL to display pool names

Outgoing mails for ZFS pool events include the pool GUID,
but not the actual pool name. Let's change this for better
readability, as it is already done in the mails for finished
pool resilvers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Marcel Menzel <mail@mcl.gg>
Closes #14272