tests/qtest/device-plug-test: skip pc tests when 'pc' machine is unavailable

test_pci_unplug_request() and test_pci_unplug_json_request() use
"-machine pc" on i386/x86_64. When QEMU is built without the i440fx/pc
machine, these fail with "unsupported machine type 'pc'".

Skip the x86 case when 'pc' is not available. Non-x86 architectures use
the default machine and are unaffected, and x86 unplug coverage is still
provided by the q35 variant (test_q35_pci_unplug_request), which already
guards on qtest_has_machine("q35").

Signed-off-by: Rohitashv Kumar <roohiit@amazon.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>