shared.cfg: Add qcow3 support

There is a 'compat' option in qcow2 image format, when it's
set to 1.1, qemu-img program will generate a 'qcow3' format
disk image.

Note: how to check whether an image is in qcow3 format:
"""
$ dd if=storage.qcow3 bs=16 count=1 2>/dev/null | hexdump
0000000 4651 fb49 0000 0300 0000 0000 0000 0000
0000010
"""

The seventh byte of qcow3 image is "0x03", and qcow2's seventh
byte is 0x02:

"""
$ dd if=storage.qcow3 bs=16 count=1 2>/dev/null | hexdump
0000000 4651 fb49 0000 0200 0000 0000 0000 0000
0000010
"""

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>