s390/protvirt: use scnprintf() instead of snprintf()

s390/protvirt: use scnprintf() instead of snprintf()

snprintf() returns the number of bytes that would be written,
which may be greater than the the actual length to be written.

uv_query_facilities() should return the number of bytes printed
into the buffer. This is the return value of scnprintf().
The other functions are the same.

Link: https://lkml.kernel.org/r/20200509085608.41061-4-chenzhou10@huawei.com
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>