nodedev: driver: Introduce internal mdevctl commands enum

nodedev: driver: Introduce internal mdevctl commands enum

This is not a 1:1 mapping to mdevctl commands because mdevctl doesn't
support a separate 'create' command. mdevctl uses 'start' for both
starting a pre-defined device as well as for creating and starting a new
transient device. The libvirt code will be more readable if we treat
these as separate commands. When we need to actually execute mdevctl,
the 'create' command will be translated into the appropriate 'mdevctl
start' command.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>