qemu-img: omit errno value in error message

qemu-img: omit errno value in error message

I'm getting io-qcow2-244 test failure on mips*
due to output mismatch:

  Take an internal snapshot:
 -qemu-img: Could not create snapshot 'test': -95 (Operation not supported)
 +qemu-img: Could not create snapshot 'test': -122 (Operation not supported)
  No errors were found on the image.

This is because errno values might be different across
different architectures.

This error message in qemu-img.c is the only one which
prints errno directly, all the rest print strerror(errno)
only.  Fix this error message and the expected output
of the 3 test cases too.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Message-ID: <20230811110946.2435067-1-mjt@tls.msk.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>