nvme: consolidate nvme requirements based on transport type

nvme: consolidate nvme requirements based on transport type

Right now, only pci and loop have tests, hence these are
the only ones that are allowed. The user can pass an env
variable nvme_trtype and check for the necessary modules.

This allows prepares us to support other transport types.

Note that test 031 is designed to run only with nvme, hence
it overrides the environment variable to nvme_trtype=pci.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>