spapr: tie spapr-nvram to -pflash

spapr-nvram's drive property is currently connected to a non-existent
"-machine nvram=<drivename>" option.  Instead, tie it to -pflash like
other non-volatile RAM devices.  This provides the following possibilities
for adding a backend for the sPAPR non-volatile RAM:

* -pflash filename

* -drive if=pflash,file=filename,format=raw,...

* -drive if=none,file=filename,format=raw,id=foo,... -global spapr-nvram.drive=foo

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>