vfio/pci: Virtualize Maximum Read Request Size

vfio/pci: Virtualize Maximum Read Request Size

BugLink: https://launchpad.net/bugs/1732804
MRRS defines the maximum read request size a device is allowed to
make.  Drivers will often increase this to allow more data transfer
with a single request.  Completions to this request are bound by the
MPS setting for the bus.  Aside from device quirks (none known), it
doesn't seem to make sense to set an MRRS value less than MPS, yet
this is a likely scenario given that user drivers do not have a
system-wide view of the PCI topology.  Virtualize MRRS such that the
user can set MRRS >= MPS, but use MPS as the floor value that we'll
write to hardware.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
(cherry picked from commit cf0d53ba4947aad6e471491d5b20a567cbe92e56)
Acked-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Manoj Iyer <manoj.iyer@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>