target/user: PGR Support

This adds initial PGR support for just TCMU, since tcmu doesn't
have the necessary IT_NEXUS info to process PGR in userspace,
so have those commands be processed in kernel.

HA support is not available yet, we will work on it if this patch
is acceptable.

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>