x86/hvm: correct hvm_ioreq_server_alloc_rangesets() failure path

x86/hvm: correct hvm_ioreq_server_alloc_rangesets() failure path

Coverity-ID: 1220092 "Unsigned compare against 0"
Coverity-ID: 1220093 "Out-of-bounds read"

Both of these are cased by the the while() loop in the fail path, which
results in an infinite loop and memory corruption from rangeset_destroy().

Move hvm_ioreq_server_free_rangesets() up and use it for cleanup on the
failure path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>