ivshmem: fix incorrect error handling in ivshmem_recv_msg()

Screwed up in commit 3a55fc0f, v2.6.0.

If qemu_chr_fe_read_all() returns -EINTR the do {} statement continues and the
n accumulator used to complete reads upto sizeof(msg) is decremented by 4 (the
value of EINTR on Linux).
To avoid that, use simpler if() statements and continue if EINTR occured.

hw/misc/ivshmem.c:650:14: warning: Loss of sign in implicit conversion
    } while (n < sizeof(msg));
             ^

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>