qemu: add option to disable X grabs

qemu: add option to disable X grabs

When the mouse pointer enters the qemu window it takes a pointer grab. This
doesn't sound too dangerous at first but it turns out that SDL will infinitely
busy-loop if it can't get the grab (e.g. if the screen is locked) and the
average autobuilder setup's X server will have locked the screen a few minutes
after boot.

The result is that on many autobuilders apparently random qemu instances (the
top-most one under the pointer) will hang during boot.

To resolve this add an option (via an environment variable) to never attempt a
grab.  The default behaviour remains to grab so that everyone else doesn't see
any change.

(From OE-Core rev: a60b1ebbb8f81245f3ccf25b3f9d63677de75b85)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>