xenfb: map framebuffer read-only and handle unmap errors

The framebuffer is needlessly mapped (PROT_READ | PROT_WRITE), map it
PROT_READ instead.

The framebuffer is unmapped by replacing the framebuffer pages with
anonymous shared memory, calling mmap. Check for return errors and print
a warning.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
