When possible, make the disk image format explicit when invoking tools
or using add-drive. This avoids warnings from qemu about the unspecified
format for the image, and also makes qemu slightly faster (skipping the
disk image probing).
Tests checking the image probing are not touched.
This changes also:
- old-style invocations of tools (`$tool $filename`) into new style
(`$tool -a $filename`)
- add-drive-ro/add-drive-with-if guestfish commands into add/add-drive
with explicit readonly/iface arguments
There should be no change in the tests results.
64bb9edd52