KVM: x86: ensure emulator fetches do not span multiple pages

When the CS base is not page-aligned, the linear address of the code could
get close to the page boundary (e.g. 0x...ffe) even if the EIP value is
not.  So we need to first linearize the address, and only then compute
the number of valid bytes that can be fetched.

This happens relatively often when executing real mode code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>