iotests/178: Pass value to invalid option

ccd3b3b811 has deprecated short-hand boolean options (i.e., options
with values).  All options without values are interpreted as boolean
options, so this includes the invalid option "snapshot.foo" used in
iotest 178.

So after ccd3b3b811, 178 fails with:

  +qemu-img: warning: short-form boolean option 'snapshot.foo' deprecated
  +Please use snapshot.foo=on instead

Suppress that deprecation warning by passing some value to it (it does
not matter which, because the option is invalid anyway).

Fixes: ccd3b3b811
       ("qemu-option: warn for short-form boolean options")
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210126123834.115915-1-mreitz@redhat.com>

tests/qemu-iotests/178

@@ -57,7 +57,7 @@ $QEMU_IMG measure --image-opts # missing filename
 $QEMU_IMG measure -f qcow2 # missing filename
 $QEMU_IMG measure -l snap1 # missing filename
 $QEMU_IMG measure -o , # invalid option list
-$QEMU_IMG measure -l snapshot.foo # invalid snapshot option
+$QEMU_IMG measure -l snapshot.foo=bar # invalid snapshot option
 $QEMU_IMG measure --output foo # invalid output format
 $QEMU_IMG measure --size -1 # invalid image size
 $QEMU_IMG measure -O foo "$TEST_IMG" # unknown image file format

tests/qemu-iotests/178.out.qcow2

@@ -11,7 +11,7 @@ qemu-img: --image-opts, -f, and -l require a filename argument.
 qemu-img: --image-opts, -f, and -l require a filename argument.
 qemu-img: Invalid option list: ,
 qemu-img: Invalid parameter 'snapshot.foo'
-qemu-img: Failed in parsing snapshot param 'snapshot.foo'
+qemu-img: Failed in parsing snapshot param 'snapshot.foo=bar'
 qemu-img: --output must be used with human or json as argument.
 qemu-img: Invalid image size specified. Must be between 0 and 9223372036854775807.
 qemu-img: Unknown file format 'foo'

tests/qemu-iotests/178.out.raw

@@ -11,7 +11,7 @@ qemu-img: --image-opts, -f, and -l require a filename argument.
 qemu-img: --image-opts, -f, and -l require a filename argument.
 qemu-img: Invalid option list: ,
 qemu-img: Invalid parameter 'snapshot.foo'
-qemu-img: Failed in parsing snapshot param 'snapshot.foo'
+qemu-img: Failed in parsing snapshot param 'snapshot.foo=bar'
 qemu-img: --output must be used with human or json as argument.
 qemu-img: Invalid image size specified. Must be between 0 and 9223372036854775807.
 qemu-img: Unknown file format 'foo'