Add xenpic_dynirq_disable_intr and set it as the .pic_disable_intr method

Add xenpic_dynirq_disable_intr and set it as the .pic_disable_intr method
for xenpic_dynirq_template.  This fixes a panic when a virtual disk is
removed, since that results in an interrupt channel being disabled and
NULL isn't very good function for disabling interrupts.

We should probably have a xenpic_pirq_disable_intr as well; I'm not adding
that here because (a) I'm not sure what uses pirqs so I don't have a test
case, and (b) the xenpic_pirq_enable_intr code is significantly more
complex than the xenpic_dynirq_enable_intr code, so I'm not sure what
should go into a xenpic_pirq_disable_intr routine.

PR:		kern/153511
MFC after:	3 days