block: Bugfix 'format' and 'snapshot' used in drive option

block: Bugfix 'format' and 'snapshot' used in drive option

When use -drive file='xxx',format=qcow2,snapshot=on the error
message "Can't use snapshot=on with driver-specific options"
can be show, and fail to start the qemu.

This should not be happened, and there is no file.driver option
in qemu command line.

It is because the commit 74fe54f2a1b5c4f4498a8fe521e1dfc936656cd4,
it puts 'driver' option if the command line use 'format' option.

This patch is to solve this bug.

Signed-off-by: Mike Qiu <qiudayu@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>