tools/libxc: Improve xc_dom_malloc_filemap() error handling

tools/libxc: Improve xc_dom_malloc_filemap() error handling

Coverity ID 1055563

In the original function, mmap() could be called with a length of -1 if the
second lseek failed and the caller had not provided max_size.

While fixing up this error, improve the logging of other error paths.  I know
from personal experience that debugging failures function is rather difficult
given only "xc_dom_malloc_filemap: failed (on file <somefile>)" in the logs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>