Switch to contigmalloc in the Hyper-V code

In the Hyper-V drivers we need to allocate buffers shared between the
host and guest. This memory has been allocated with bus_dma, however
it doesn't use this correctly, e.g. it is missing calls to
bus_dmamap_sync. Along with this on arm64 we need this memory to be
mapped with the correct memory type that bus_dma may not use.

Switch to contigmalloc to allocate this memory as this will correctly
allocate cacheable memory.

Reviewed by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D40227