tests: specify the image format when possible

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.

fuse/test-fuse-umount-race.sh

@@ -57,7 +57,7 @@ guestfish -- \
       backingfile:../test-data/phony-guests/fedora.img backingformat:raw
 
 mkdir mp
-./guestmount -a test.qcow2 -m /dev/VG/Root --pid-file test.pid mp
+./guestmount --format=qcow2 -a test.qcow2 -m /dev/VG/Root --pid-file test.pid mp
 cp $0 mp/test-umount
 
 # Save the PID of guestmount.
@@ -82,7 +82,7 @@ fi
 # It should now be safe to copy and read the disk image.
 cp test.qcow2 test-copy.qcow2
 
-if [ "$(guestfish -a test-copy.qcow2 --ro -i is-file /test-umount)" != "true" ]; then
+if [ "$(guestfish --format=qcow2 -a test-copy.qcow2 --ro -i is-file /test-umount)" != "true" ]; then
     echo "$0: test failed"
     exit 1
 fi