KVM: x86: do not fail if software breakpoint has already been removed

KVM: x86: do not fail if software breakpoint has already been removed

If kvm_arch_remove_sw_breakpoint finds that a software breakpoint does not
have an INT3 instruction, it fails.  This can happen if one sets a
software breakpoint in a kernel module and then reloads it.  gdb then
thinks the breakpoint cannot be deleted and there is no way to add it
back.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>