qemu.test: make block_stream test work with qmp monitor

Fix qemu-img create snapshot file failed when image format is raw
Corrected block stream command in human monitor (block-stream ->
block_stream)

changes from v1:
    Removed configuration to set QMP as default monitor

Signed-off-by: Xu Tian <xutian@redhat.com>
qemu.tests: move snapshot file to images dir

Signed-off-by: Xu Tian <xutian@redhat.com>