plugin-base: new 'allowed-drivers' property

The plugins can set this property to filter support check requests by physical
device driver. The value given to the property should be a NULL-terminated array
of C strings, e.g.:

    const gchar *drivers[] = { "qcserial", NULL };

src/mm-plugin-base.h

@@ -126,6 +126,7 @@ typedef struct {
 #define MM_PLUGIN_BASE_NAME                    "name"
 #define MM_PLUGIN_BASE_ALLOWED_CAPABILITIES    "allowed-capabilities"
 #define MM_PLUGIN_BASE_ALLOWED_SUBSYSTEMS      "allowed-subsystems"
+#define MM_PLUGIN_BASE_ALLOWED_DRIVERS         "allowed-drivers"
 #define MM_PLUGIN_BASE_ALLOWED_VENDOR_IDS      "allowed-vendor-ids"
 #define MM_PLUGIN_BASE_ALLOWED_PRODUCT_IDS     "allowed-product-ids"
 #define MM_PLUGIN_BASE_ALLOWED_VENDOR_STRINGS  "allowed-vendor-strings"