CA-253489 xe update-upload: test if default SR is valid
CA-253489 xe update-upload: test if default SR is valid

if no sr-uuid parameter was specified, as in this case the command falls
back to the pool's default SR. Now an error will be shown to the user in
case of a null/invalid default SR, and we'll fail early, instead of
failing later in the import_raw_vdi handler.

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

srowe/xen-api,euanh/xen-api,euanh/xen-api,srowe/xen-api,srowe/xen-api,euanh/xen-api,euanh/xen-api,euanh/xen-api,srowe/xen-api,euanh/xen-api,srowe/xen-api