build: fix build without i18n

build: fix build without i18n

If you compile without NLS support, where _() is a no-op macro,
then we end up passing a string literal to a char*, provoking:

In file included from virsh.c:3639:0:
virsh-edit.c: In function ‘cmdSaveImageEdit’:
virsh-edit.c:97:13: error: assignment discards ‘const’ qualifier from pointer target type [-Werror]
virsh-edit.c:106:13: error: assignment discards ‘const’ qualifier from pointer target type [-Werror]

* tools/virsh-edit.c: Be const-safe.