power: fix error messages

power: fix error messages

Function strerror(errno) has built strings only for non-negative errno values.
for negative values of errno it describe error as "Unknown error -errno"
to be more descriptive i put string "channel not found" taken from header.

The negative argument will be interpreted as a very large unsigned value.

Coverity issue: 13266
Coverity issue: 13269
Fixes: 445c6528b55f ("power: common interface for guest and host")

Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>