qemu: Adjust max memlock on mdev hotplug

qemu: Adjust max memlock on mdev hotplug

When starting a domain, we use the presence of a vfio-pci or
mdev hostdev to determine if the memlock maximum needs to be
increased.  But if we hotplug either of these devices, only the
vfio-pci path gets that love.  This means that attaching a, say,
vfio-ccw device will appear to succeed but the device may be
unusable as the guest may see I/O errors on long CCW chains.
The host, meanwhile, would be flooded with these messages:

  vfio_pin_page_external: Task qemu-system-s39 (11584) RLIMIT_MEMLOCK (65536) exceeded

Let's adjust the maximum memlock value in the mdev hotplug path,
so that the domain has the same value as if it were started with
one or more mdev devices in its configuration.

Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>