xenlight: when using upstream qemu, we must use SEABIOS
xenlight: when using upstream qemu, we must use SEABIOS

Previously we hard-coded ROMBIOS, which causes libxl to fail silently.
Instead we let libxl choose the default for us.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

cheng--zhang/xenopsd,koushikcgit/xenopsd,johnelse/xenopsd,euanh/xenopsd,johnelse/xenopsd,cheng--zhang/xenopsd,simonjbeaumont/xenopsd,djs55/xenopsd-previous,euanh/xenopsd,djs55/xenopsd-previous,koushikcgit/xenopsd,djs55/xenopsd,thomassa/xenopsd,thomassa/xenopsd,djs55/xenopsd,koushikcgit/xenopsd,euanh/xenopsd,thomassa/xenopsd,simonjbeaumont/xenopsd,simonjbeaumont/xenopsd,cheng--zhang/xenopsd,djs55/xenopsd,djs55/xenopsd-previous,johnelse/xenopsd