hw/arm/virt: Rename default_bus_bypass_iommu

Since commit d8fb7d09 ("vl: switch -M parsing to keyval"), machine
parameter definitions cannot use underscores, because keyval_dashify()
transforms them to dashes and the parser doesn't find the parameter.

This affects option default_bus_bypass_iommu which was introduced in the
same release:

$ qemu-system-aarch64 -M virt,default_bus_bypass_iommu=on
qemu-system-aarch64: Property 'virt-6.1-machine.default-bus-bypass-iommu' not found

Rename the parameter to "default-bus-bypass-iommu". Passing
"default_bus_bypass_iommu" is still valid since the underscore are
transformed automatically.

Fixes: 6d7a8548 ("hw/arm/virt: Add default_bus_bypass_iommu machine option")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Tested-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211026093733.2144161-1-jean-philippe@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>