qemu-iotests: map underscore to dash in QMP argument names

qemu-iotests: map underscore to dash in QMP argument names

iotests.py provides a convenience function that uses Python keyword
arguments to represent QMP command arguments.  However, almost all
QMP commands use dashes for argument names (the sole exception is
block_set_io_throttle), and dashes are not allowed in a keyword
argument name.  Hence provide automatic conversion of underscores
to dashes.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>