changes to be committed, although qemu can support this.
This is equivalent to the qemu parameter
-C<-drive file=filename,snapshot=on,if=...>.
+C<-drive file=filename,snapshot=on,readonly=on,if=...>.
C<if=...> is set at compile time by the configuration option
C<./configure --with-drive-if=...>. In the rare case where you
might need to change this at run time, use C<guestfs_add_drive_with_if>
or C<guestfs_add_drive_ro_with_if>.
+C<readonly=on> is only added where qemu supports this option.
+
Note that this call checks for the existence of C<filename>. This
stops you from specifying other types of drive which are supported
by qemu such as C<nbd:> and C<http:> URLs. To specify those, use