xl: make return type of create_domain() more consistent.

xl: make return type of create_domain() more consistent.

create_domain() is of uint32_t return type, because on
success it returns the domid of the new domain, and
uint32_t is what we typically use for domid-s.

However, on failure, it returns ERROR_FAIL or ERROR_INVAL,
which are -3 and -6. Callers assign the return value to an
'int rc' variable and then check for '(rc < 0)'.

Although things work, and no tool (compiler, Coverity, ecc.)
is complaining, using 'int' as return type seems better.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>