linux-user: fix emulation of /proc/self/maps

Improve the emulation of /proc/self/maps by reading the underlying
host maps file and passing lines through with addresses adjusted
to be guest addresses. This is necessary to avoid false triggers
of the glibc check that a format string containing '%n' is not in
writable memory. (For an example see the bug reported in
https://bugs.launchpad.net/qemu-linaro/+bug/947888 where gpg aborts.)

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
