-(dom0-min-mem 256)
+(dom0-min-mem 240)
あ、イミナシ。馬鹿杉。
libvir: Xen Daemon error : POST operation failed:
(xend.err 'Error creating domain: I need 262144 KiB,
but dom0_min_mem is 245760 and shrinking to 245760 KiB would leave only 260032 KiB free.')
(dom0-min-mem 232)にしてみて、再起動。