[XEND] Fix fallback and error path for domain destruction by domid,

[XEND] Fix fallback and error path for domain destruction by domid,
or of non-existent domain.

I saw the following error message. (A domain of domain name XXX does
not exist.)

# xm list
Name                              ID Mem(MiB) VCPUs State  Time(s)
Domain-0                           0     1019     2 r-----    29.8
# xm destroy XXX
Error: an integer is required

This patch fixes it to print the following error message.

# xm list
Name                              ID Mem(MiB) VCPUs State  Time(s)
Domain-0                           0     1019     2 r-----    35.8
# xm destroy XXX
Error: the domain 'XXX' does not exist.

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>