qemu-iotests: Discard specific info in _img_info

qemu-iotests: Discard specific info in _img_info

In _img_info, filter out additional information specific to the image
format provided by qemu-img info, since tests designed for multiple
image formats would produce different outputs for every image format
otherwise.

In a human-readable dump, that new information will always be last for
each "image information block" (multiple blocks are emitted when
inspecting the backing file chain). Every block is separated by an empty
line. Therefore, in this case, everything starting with the line "Format
specific information:" up to that empty line (or EOF, if it is the last
block) has to be stripped.

The JSON dump will always emit pretty JSON data. Therefore, the opening
and closing braces of every object will be on lines which are indented
by exactly the same amount, and all lines in between will have more
indentation. Thus, in this case, everything starting with a line
matching the regular expression /^ *"format-specific": {/ until /^ *},?/
has to be stripped, where the number of spaces at the beginning of the
respective lines is equal.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>