x86: xen: platform-pci-unplug: use pr_err() and pr_warn() instead of raw printk()

Since we have the nice helpers pr_err() and pr_warn(), use them instead
of raw printk().

[jgross@suse.com] Move the "#define pr_fmt" above the #includes in
                  order to avoid build warnings due to redefinition.

Signed-off-by: zhaoxiao <zhaoxiao@uniontech.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210825114111.29009-1-zhaoxiao@uniontech.com


Signed-off-by: Juergen Gross <jgross@suse.com>