xen: wait up to 5 minutes for device connetion

Increases the device timeout from 10s to 5 minutes, giving the user a
visual indication during that time in case there are problems.  The patch
is a backport of changesets 144 and 150 in the Xenbits tree.

Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>