tmem: check the return value of copy to guest

tmem: check the return value of copy to guest

Use function copy_to_guest_offset/copy_to_guest directly and check their return
value.

This also fixes CID 1132754, and 1132755:
"Unchecked return value
If the function returns an error value, the error value may be mistaken for a
normal value.  In tmem_copy_to_client_buf_offset: Value returned from a function
is not checked for errors before being used (CWE-252)"

And CID 1055125, 1055126, 1055127, 1055128, 1055129, 1055130
"Unchecked return value
If the function returns an error value, the error value may be mistaken for a
normal value.  In <functions changed>: Value returned from a function is not
checked for errors before being used (CWE-252)"

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>