um-virt-pci: Make max delay configurable

There is a hard coded maximum time for which the driver busy-loops
waiting for a response from the virtio device.  This default time may be
too short for some systems, so make it a module parameter so that it can
be set via the kernel command line.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>