This is a bug which was introduced in
a1c46d139b79ad7f27ab2dee6f6ff58ac9de65d1

This is no 'qtree' keyword in the output of any
version of qemu, thus the method to check if
qemu supports qtree in that commit would disable
qtree check for every qemu binary.

This commit tries to run the 'info qtree' command
directly, if everything goes well, it means we
have qtree support.
Reported-by: NMike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>