bhyve: refactor NVMe PRP memcpy

Add a "copy direction" parameter to nvme_prp_memcpy such that data can
be copied to the memory specified by the PRP entries (current behavior)
or copied from the PRP entries (new behavior). The upcoming deallocate
functionality will use the copy from capability.

Reviewed by: jhb, araujo
Approved by: jhb (maintainer)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21837