xen: Rename asprintf() to xasprintf()

xen: Rename asprintf() to xasprintf()

Coverity reports that there is a memory leak in
ioreq_server_alloc_rangesets().  This would be true if Xen's implementation of
asprintf() had glibc's return semantics, but it doesn't.

Rename to xasprintf() to reduce confusion for Coverity and other developers.

While at it, fix style issues.  Rearrange ioreq_server_alloc_rangesets() to
use a tabulated switch statement, and not to have a trailing space in the
rangeset name for an unknown range type.

Coverity-ID: 1472735
Coverity-ID: 1500265
Fixes: 780e918a2e54 ("add an implentation of asprintf() for xen")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>