hyperv/vmbus: Free message taskqueue during interrupt teardown

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6506

hyperv/vmbus: Check hyperv_dmamem_alloc return value

Though it is highly unlikely this function would fail w/ BUS_DMA_WAITOK,
we had better to check its return value; better safe then sorry here.

MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D6518