bhyve: refactor NVMe namespace initialization

Pass the struct pci_nvme_blockstore pointer for this namespace to the
namespace initialization function instead of only the desired eui64
value.

Minor functional change in that the code updates the eui64 value in the
blockstore.

Reviewed by: jhb, araujo
Approved by: jhb (maintainer)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21838