Solaris format command fails with Warning: error setting drive geometry.Warning: error writing VTOC.Label failed.

The entry in /devices/pseudo need to be recreated. First of all, check that the issue is not caused by the presence of the HPOvPerfAgt package. Otherwise, use one of the 4 following methods. If the 1st method fails, the second method should succeed. In the worst-case scenario,  the last one should succeed.

1) try to recreate the emcp entries in /devices/pseudo using the "devfsadm -i emcp" command
# devfsadm -i emcp
# ls /devices/pseudo/emcp@0     (check that the entries now exist - note that emcp@0 is for emcpower0; for emcpowerXX, look for emcp@XX)
# bootadm update-archive    (to consolidate the configuration change in the boot information) 

2) The previous command might be unsuccessful in creating the new entries in /devices. The command below is based on the fact that, when a successful "emcpadm renamepseudo" command is run, all the entries in /devices are rebuilt. This is a side-effect of this command which will be very useful to fix the issue here.
Pick-up an unused pseudo device (not currently mounted and part of a filesystem)  which is not having any issue (which is showing up in format and for which entries exist under /devices). Let's say this good pseudo-device is emcpower7. And let's assume that there is no emcpower1007 in the configuration so that emcpower1007 is a possible new name. Run :
# emcpadm renamepseudo -s 7 -t 1007
# emcpadm renamepseudo -s 1007 -t 7
# powermt save
# bootadm update-archive

It is important to use a "good" pseudo device .... and not the pseudo device which has no entry in /devices otherwise the command will fail (see paragraph 4 below) and will not fix the /devices entries.
With the above commands, after the first one, the issue should already be fixed. The second rename is just there to get the original pseudoname back. The last 2 commands are there to consolidate the change.

3) try to unmanage / manage the problematic device (this will probably fail)
# powermt unmanage dev=emcpower0a
ERROR: Unmanage failed

4) try to rename the problematic pseudo device  (this will probably fail)
# emcpadm renamepseudo -s 0 -t 1000
<ERROR: can not rename device. emcpower0 does not exist> 

5) completely reconfigure the problematic device
# powermt remove dev=0
# powermt save
>> unmask the device from the Storage Group
# devfsadm -C (to remove the entries in /dev/dsk and /dev/rdsk for the native paths)
# rm /dev/dsk/emcpower0*
# rm /dev/rdsk/emcpower0*
>> mask the device back into the Storage Group
# devfsadm (to recreate the entries for the native paths)
# powermt config (to reconfigure Powerpath)
# powermt display dev=all (check that the pseudo device was reconfigured)
# powermt save
# ls /devices/pseudo/emcp@0     (check that the entries now exist - note that emcp@0 is for emcpower0; for emcpowerXX, look for emcp@XX)
# bootadm update-archive    (to consolidate the configuration change in the boot information)