VT-d: decouple SandyBridge quirk from VTd timeout

Currently the quirk code for SandyBridge uses the VTd timeout value when
writing to an IGD register.  This is the wrong timeout to use and, at
1000 msec., is also much too large.  This patch changes the quirk code
to use a timeout that is specific to the IGD device and allows the user
control of the timeout.

Boolean settings for the boot parameter `snb_igd_quirk' keep their current
meaning, enabling or disabling the quirk code with a timeout of 1000 msec.

In addition specifying `snb_igd_quirk=default' will enable the code and
set the timeout to the theoretical maximum of 670 msec.  For finer control,
specifying `snb_igd_quirk=n', where `n' is a decimal number, will enable
the code and set the timeout to `n' msec.

Signed-off-by: Don Dugger <donald.d.dugger@intel.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>

Update documentation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>