tests/qtest/libqos: Skip hotplug tests if pci root bus is not hotpluggable

ARM does not not support hotplug on pcie.0. Add a flag on the bus
which tells if devices can be hotplugged and skip hotplug tests
if the bus cannot be hotplugged. This is a temporary solution to
enable the other pci tests on aarch64.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20220504152025.1785704-3-eric.auger@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>