qemu: rdp: Fix 'qemuRdpAvailable()'

qemu: rdp: Fix 'qemuRdpAvailable()'

qemuRdpAvailable() is called from the capability filing code, thus:
- it must not report spurious errors
- it should not call any extra processes

We can solve the above by just checking existance of 'qemu-rdp' in the
path as:
- at the time of adding of qemuRdpAvailable() there was only one 'qemu-rdp' release
- it supported all the features
- the check can't change as we'd drop the capability

Add comments and gut the check to only check existance of the file.

Fixes: f5e5a9bec9ec3e6c762f5000e3b8a0ba6a3a8c8d
Closes: https://gitlab.com/libvirt/libvirt/-/issues/763
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>