linux-user: Use init_guest_space when -R and -B are specified

Roll the code used to initialize the guest memory space when -R
or -B is used into 'init_guest_space' and then call 'init_guest_space'
from the driver.  This way the reserved guest memory space can
be probed for.  Calling 'mmap' just once as is currently done is not
guaranteed to succeed since the host address space validation might fail.

Signed-off-by: Meador Inge <meadori@codesourcery.com>
[PMM: Fixed minor whitespace errors.]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
