bhyve: refactor NVMe I/O read/write

Split the NVM I/O function (i.e. nvme_opc_write_read) into separate
functions - one for RAM based backing-store and another for disk based
backing-store for easier maintenance. No functional changes.

Tested by: Jason Tubnor
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24890